tags send the browser haywire if they are being viewed using Netscape 4, and then the page is resized. The only solution is to detect when the browser is resized, and then refresh the page to clean it up. There are other reasons you may want to reload the page. For example, you may want to create your own navigation buttons. Regardless of the reasons, the location object provides a method with which to do it. To reload the current page, simply use the reload() method as shown below: location.reload();

Note that the reload() method is similar to holding down the Shift key and hitting the reload button. Simply clicking on refresh will leave data that has been entered into any HTML form elements intact, but the Shift+Refresh key combination clears everything. This is exactly what the reload() method of the location object does. It does a complete reload of the page, which normally should be from the browser’s cache. We say “should” because browser implementations frequently don’t implement the reload() method as they are supposed to do. For an even fuller refresh, add the logical value true as an argument to the reload() method: location.reload(true);

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This should not only refresh the page but also go to the server to collect the most up-todate version. Again, however, we say “should” because this isn’t always what happens. Hopefully in the future browsers will behave a bit more consistently, but for the moment there is a workaround for the times when you really need the newest version of a page to load. This involves adding a timestamp to the end of the page as a query string. We will look at how to generate a timestamp later when we deal with the Core JavaScript Date object in Chapter 9, “Date and Time Manipulation,” but for the moment let’s continue with our overview of JavaScript objects. There is also a method that is designed to reload the page from the cache and leave form elements filled out. It belongs to the window object’s history object.

The history Object You likely are aware that Web browsers store a list of the pages you have visited in what is called the browser’s history. This enables the browser to supply back and forward buttons to allow users to go backward and forward through the sites they have recently visited. You already have limited access to the browser’s history through the window object’s history object. Under normal conditions, it isn’t possible to discover the URLs of the sites where a user has been; however, the history object allows you to do simple things such as move backward and forward within the browser’s history. The history object has the length property and the following methods: back() forward() go()

Netscape provides an additional three properties—current, next, and previous—that provide the URIs of the current, next, and previous history entries respectively. They all require signed scripts, which we will cover in detail later in the book. The history object’s length property stores the number of history events in the browser’s history. In reality, this is only of use in conjunction with signed scripts when history entries can be obtained. Without that information, you can’t determine where you are in the history events and, thus, which of the other entries you may wish to move to. It is really the methods of the history object that are the most useful. The back and forward buttons are fairly self-explanatory. They allow you to force the browser back to the last page that was loaded or forward to the next. If you wanted to create your own back and forward buttons, you could use hyperlinks with these methods to achieve the effect.

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Just remember that there will need to be history entries for before and after the current page for the buttons to work! See Listing 4.5. LISTING 4.5

Simulated Back and Forward Buttons (backAndForward.htm)

Simulated Back and Forward Buttons

Back | Forward

To create more attractive buttons, simply wrap the tags around some images instead of the text. When moving forward or backward by one is not enough you can use the go() method. This allows you to specify how many steps backward or forward in the history the browser should go. For example to go back to the previous page, write this: window.history.go(-1);

Or to go forward by two you would write this: window.history.go(2);

If you overshoot the entries, even by one, then the go() method is ignored. Using the go() method in this way is problematic as you most likely will need to have access to the history entries for your scripts to determine where to go. However, what the go() method is very useful for is refreshing a page without removing any data that a user may have entered into an HMTL form. This is opposed to the fuller refreshes provided by the location object’s reload() method. To do this more limited type of refresh, you would write this: window.history.go(0)

The 0 just means that it is the present history entry—that is, the present document—that should be loaded. Just “soft” refresh.

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Note that it is the history object that is used to effect a soft refresh, and it is the location object that is used to do a hard refresh. Since the back() and forward(), and go() methods belong to the history object it is easy to forget that the reload() method belongs to the location object and thus make mistakes in your scripts.

The navigator Object Netscape was the first company to provide a mechanism for finding out information about the browser being used to view a Web page. It was quite logical then that the object that was introduced to provide scripts with information about Netscape Navigator was given the name navigator. At the time, Netscape was king in the browser world and so other browser manufacturers gave the object in their browsers the same name so the JavaScript being used on the Web also would work in their browsers. In Internet Explorer, the navigator object can also be accessed using the name clientInformation. However in Internet Explorer both names point (refer) to the same object. Because clientInformation works only in Internet Explorer but navigator works in both main browsers, the norm is to use the name navigator to access the navigator object. This ensures that your scripts will work on all browsers. The navigator object has many properties. Because several of the navigator object’s properties are specific to individual browsers, only cross-browser properties and some of the more useful proprietary properties are listed: appCodeName appName appVersion browserLanguage cookieEnabled language platform systemLanguage userAgent userLanguage

The main use of these properties is to provide our scripts with information that will enable them to determine whether the browser a visitor to our site is using will support

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certain pieces of code we may want to use. This information can be used in conjunction with the control statements that we saw in the previous chapter to control whether the code is executed or not. Unfortunately the information that some of these properties provide can be quite misleading and difficult to understand. We will discover how to use them later in Chapter 8, “The Browser Issue.”

The screen Object The screen object provides our scripts with useful information about the size and settings of the monitor that a visitor to our site is using. This can be useful when we want to move an existing or new window to a certain place on the screen, or when we need to know whether the screen has high enough resolution or color depth to support something we want to use. The properties that are widely supported are: availHeight availWidth colorDepth height pixelDepth width

There are also two other very useful properties, availLeft and availTop, which surprisingly are not supported by Internet Explorer. A more in-depth study of the screen object will be discussed in Chapter 12, “Windows and Frames,” where it is more applicable.

Core JavaScript Objects Core JavaScript, the parts of JavaScript that exist in every environment in which it is used, is made up of several language elements. You have already learned about most of the operators and statements that are available, but just as important are the Core JavaScript objects. Without these objects, what you can do is severely limited. There are 11 Core JavaScript objects, which have been listed below: Array Boolean Date Error Function

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Global Math Number Object RegExp String

In the client-side environment the Global object is, of course, the window object. You have already used the window object to discover how objects are accessed and in so doing have seen some of its properties and methods. However, learning about the window object is a important topic, so we will take a closer look at it later, mostly in Chapter 12. All the other Core JavaScript objects (which in the client-side browser environment belong to the window object) are also important topics in their own right, and much of the rest of this book will be spent discovering more about them. The remainder of this chapter will take an introductory look at what these objects are and how to make use of them. The Core JavaScript objects are very different to the other objects that belong to the window object. For one thing, the majority of the time you will not use the Core JavaScript objects directly in the way you have used the other objects. More frequently, you will be using copies, or clones, of these objects that JavaScript has automatically created or that you have created for yourself. In general, you can create a copy of one of the Core JavaScript objects using one of two ways. You can create it either by using literal notation or by using constructor function notation. You already have used literal notation on many occasions. For example, whenever you have created a string, it has been based on the String object. You didn’t have to do anything special; you simply surrounded the characters for the string with quote marks and, seeing the quote marks, JavaScript automatically created a string literal. The same happens for boolean and numerical values. JavaScript recognizes them as keywords or values and knows automatically what object on which to base them (the Boolean and Number objects respectively). When you use literal notation the result is usually a true literal, which means that it is not an object in and of itself. It simply gains access to the properties and methods of the object on which it was based. You can’t give it any properties or methods of your own. When you create an instance of an object using the constructor function notation it is quite different. Instances are full objects in their own right. They not only inherit the properties and methods of the object on which they were based, but your own properties or methods and, if you want, even your own child objects can be added.

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So far you have learned how to create your own data (which as just discussed also can be called “literals”) and some ways of working with that data in the form of operators and statements. What you haven’t learned yet are ways of structuring or ordering your data. This is one of the useful purposes of creating instances of Core JavaScript objects. It is possible to create your own objects from scratch, along with the properties and methods that will be needed for them. However this can be quite involved. To make the job a lot easier, use the Core JavaScript objects as prototypes or blueprints. They provide a way to easily make your own data structures that will have many useful management tools (methods) and properties already built in. So what is constructor function notation, and how is it used to create instances of the Core JavaScript objects? Well, to understand this, you have to understand that all of the Core JavaScript objects are also functions. At first this may seem very strange. You probably have functions and objects as totally separate entities in your mind. Actually, though, it isn’t too hard to understand. Functions can take on properties and methods (additional functions if you like) as belonging to them, as well as child objects of their own. You just haven’t seen them used in that way yet. For example you can write this: function myFunc() { alert(“hello”); }

and then write: myFunc.myProp = “another hello”;

If the function myFunc didn’t already have the property called myProp, then it would immediately be created and assigned the string literal “another hello”. Functions are objects, and although they weren’t signified by writing the call operators after their names in the list of Core JavaScript objects above, all those objects except the Math object are function objects (even the function object itself). In fact if you alert the names of these objects in an alert box, you will be told that they are functions. For example if you were to write the following:

INPUT

alert(Boolean);

you will see the alert box shown in Figure 4.9.

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OUTPUT FIGURE 4.9 The alert() function showing the character of Boolean.

ANALYSIS This is significant because you can call the Core JavaScript objects as functions

and, when you do, they will create a copy of themselves. For example you could write this: var myNum = Number();

This only creates a number literal; it doesn’t create an instance of the Number object (a full copy that is a number object) and so isn’t proper constructor function notation. To create an instance of one of the Core JavaScript objects, you have to use another of the operators, the new operator. Here is how to use it with the Number() function. var myNum = new Number();

When a Core JavaScript object is called as a function like this using the new operator it is true constructor function notation and a new copy of the object is automatically created. That’s it! You don’t have to write any methods for it or assign it any properties of your own. Most of what you usually will need is already there. You simply have to add the data and you are ready to go.

Note

Note that the Math object is the exception to this rule. You cannot make copies of the Math object at all, either literals or instances! The Math object is used directly.

Admittedly, you don’t often have a reason to create an instance of the Number object so that you can add properties and methods to it (although sometimes it can be useful). The Core JavaScript object that you most often will want to make copies of is the Array object. The Array object is an extremely powerful way of creating a data structure, and its methods provide some extremely powerful tools for managing the data it contains. For this reason, it will be investigated in Chapter 5. It is worth mentioning that when a page loads, the Core JavaScript objects are all constructed themselves. The original object is the Function object. It is then used as a constructor function to create all the Core objects except the Math object. The Object object,

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which has been created from the Function object, is used to construct the Math object. This helps to explain why some of the Core objects own some of the properties and methods that they do. Some don’t seem applicable to the object they belong to, but it is because they inherit them from the Function object. Another thing to note from this is that you can use the Function object to create your own object constructor functions, which will be discussed in detail much later in the book. As a final note on the Core JavaScript objects, each core object has its own notations for creating literals and full instances. Using constructor function notation will always result in a full instance, but literal notation will not always result in a literal. For example, when you use literal notation to create an Array or Object object, it will create a full instance anyway. This is because it is the nature of some of these objects that they can only create objects.

Exploring an Object Before this chapter ends, let’s look at two of the JavaScript operators that weren’t discussed in Chapter 2. Both of these operators are useful once you start working with objects. One, the in operator, allows you to check whether a property or child of an object exists, and the other, the typeof operator, allows you to discover the type of data or object with which you are dealing. When you use the in operator you are checking for the existence of a property, method, or child object in another object by name. For example, if you wanted to discover if there was a child object of the window object called “document” you could write this: alert(“document” in window);

// alerts true

Because the document object does contain an object with the name document the alert() function would display an alert box with the value true. Note that the name of the item we are checking for is written as a string. If we were to write its name as code, the result will simply be false. As it happens, this use of the in operator is not supported in browser versions before Internet Explorer 5 and Netscape 6. Originally, it was limited to use in a for loop. To demonstrate how it can be used in this way, let’s look at an example. For this example assume that an object called bike was created, and you assigned it two properties. One of the properties has the name color and the value green and the other has the name age and the value 5. If you wanted to show all of the bike’s properties and their values, you could use the for loop with the in operator as shown here:

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INPUT

155

var props = “”; for (var prop in bike) { props += “The bike’s “ + prop + “ is “ + bike[prop] + “\n”; } alert(props);

This would bring up an alert box with the result shown in Figure 4.10.

OUTPUT FIGURE 4.10 Using a for loop to display all the properties of the bike object.

At first how this form of the for loop works may seem hard to understand. There is no counter variable, no increment of a loop counter, and no condition. However, it isn’t actually too complicated. Basically when you use a for in loop like this, during each loop that is made the variable called prop contained within the for loop parentheses is assigned the name of a new property belonging to the object to the right of the in operator. The for statement will continue to loop until the variable prop has been assigned the names of all the properties once. Then it will stop. JavaScript manages internally the assigning of the variable names and the ending of the loop.

ANALYSIS

In the one statement that the for loop governs in our example above, the variable prop is used to build up a string containing information about the bike object. This string starts out with the value “The bike’s “ to which the property name contained in the variable prop is concatenated. To this, you then concatenate the string “ is “ before finally concatenating the value of the property to the string and a line break. The method used to obtain the value of the property is not one you have come across yet. You will learn more about this use of square brackets in the next chapter, but for now just accept that it retrieves the value of the property. As the for loop loops through the properties of the bike object, it builds up a string stored in the props variable before that string finally is alerted when the loop ends. Of course you could change the statements in the for loop so that it cycles through the properties of an object performing any process you want. Now let’s find out about our second operator.

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The typeof operator is another useful operator. It is invaluable when you need to take different actions on a piece of data depending on its data type. When applied to a piece of data this operator will return one of six strings. The possible values of these strings are listed below: boolean function number object string undefined

Notice that, even though functions are a type of object, the typeof operator will distinguish functions from other types of objects that may exist. Here are some examples of the typeof operator applied to some objects and literals: alert(typeof alert(typeof alert(typeof alert(typeof

window.document); 3); document.scrollBy); true);

// // // //

alerts alerts alerts alerts

“object” “number” “function” “boolean”

All these alerts return the string that you would hopefully expect them to. The only result that may surprise you is that if applied to the value null, then the typeof operator will return the string “object”. Although it is not often that that you will need to know this, it is worth bearing in mind. To see both the in and typeof operators in action in a real script, let’s look at a full example. In Listing 4.6, the for in loop and the typeof operators were used to expose the properties of the navigator object and to discover the values and types of these properties.

INPUT

LISTING 4.6

The navigator Object Exposed (objectExplorer.htm)

<style type=”text/css”> td {padding: 0.05em 1em}

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LISTING 4.6

157

continued

Object Explorer

<script language=”javascript” type=”text/javascript”> ”); document.write(“ ”); document.write(“ ”); document.write(“ ”); document.write(“”); } //-->

Property	Data Type	Value
” + prop + “	” + typeof obj[prop] + “	” + obj[prop] + “

The first thing we have done in this example is to start a table and give it three headings. This has been done outside the script block because we will not be repeating this process.

ANALYSIS

In the first line of script, a reference to the navigator object to a variable called obj was assigned. This saves you from writing out window.navigator three times in the for loop that follows the variable. The for loop was set up like the previous example, but this time the variable called obj was placed to the right of the in operator so that it is the navigator object that the for loop will loop through. In the statement block of the for loop, write out a new row for each loop. The first of the cells is given the name of the property that is stored in the variable prop. The second cell is a little more complex. The typeof operator was applied to the value contained within the property referred to by the variable prop. This will tell you the type of data or if it is an object contained within the property. Finally, the value of the property itself was assigned to the third cell. When loaded the page should contain a table similar to the one shown in Figure 4.11.

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OUTPUT FIGURE 4.11 Using a for in loop to explore the properties of the navigator object.

Note that the properties that belong to the navigator object vary from browser to browser so when you run the code you may be missing some of the properties shown in Figure 4.11 or have additional ones. By changing the object assigned to the variable obj you can use this page to discover the properties belonging to other objects. However when a for in loop is used to explore an object it will reveal the names of all child objects, properties, and event handlers. Therefore, if you change the above example to explore other objects, you may see names that you don’t expect. Event handlers always start with the word “on”. So for example, if you see the name onclick, what you are seeing is the name of an event handler. We will discover what event handlers are used for and how they are used in Chapter 10, “Events and Event Handling.” Note that using the for in loop will only reveal the methods of an object if it is an object that you have created yourself. When used to explore a predefined object such as the window object they will not show up.

Summary This chapter took a big step toward giving you the tools you will need to make a real difference to your Web pages. You have learned how JavaScript models everything it can work with as objects with properties and methods, as well as providing you with Core objects of its own. When used in the client-side browser environment you gain access to some of the browser and all of the document through the DOM.

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Workshop In this workshop, questions and exercises will be used to review what you have learned in this chapter about objects.

Q&A Q. In object terminology, what is equivalent to a function? A. An object has methods, which are equivalent to a function. Q. Does the window object always need to be used in the object path when we navigate to one of its child objects? A. No. From this chapter forward, you will cease to explicitly navigate to objects through the window object. Remember that because it is the default it does not need to be included in our object paths. It was used in this chapter simply to help familiarize you with navigating to objects through the hierarchy tree.

Quiz 1. Which Core JavaScript object can you not create a copy of? 2. What is the Global object called in client-side JavaScript? 3. The document object is a child of which object?

Quiz Answers 1. You can’t create a new copy of the Math object. It must be used directly. 2. The window object is the Global object on the client side. 3. The document object is a child of the window object.

Exercises 1. Display a message such as “Welcome to the world of JavaScript” in the status bar of the browser window. 2. Create an HTML page which contains JavaScript, which uses the document.write() method to write the values of the properties of the document object to the Web page.

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An Introduction to Arrays Arrays are the first JavaScript objects that we will look at in detail. This chapter will introduce the concept of arrays in JavaScript and how arrays are used, and it will show you how they are useful both in terms of structured data storage and in accessing document objects. This chapter will teach you • What arrays are • How arrays are created • How arrays are used • The difference between indexed and associative arrays

What Is an Array? An array is a special type of object that uses numbers to access each characteristic of the object—in much the same way as property names are used. If you remember what has been said about naming rules in JavaScript, you are probably thinking that this goes fully against the naming conventions discussed earlier, and you would be right. When saying the numbers are like property

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names, we don’t mean that they are exactly the same. Each number does refer to a unique piece of data belonging to the array object, but each number also is referred to as an index of the array—not a property name. This comparison between property names and indexes was made simply to help you grasp the concept of an array index. Indexes, like property names, are simply the means of accessing an array’s data containers. Since array indexes are not, technically speaking, the same as property names, you can’t use the dot notation that you would use for property names. If JavaScript names are not allowed to start with a number, then they certainly aren’t allowed to be only a number. Because the indexes of an array are numerical they have to be written in a special way. What you need to do is wrap the index (number) in square brackets. So, the code to access three individual indexes of an array would look like the following: myArray[2] myArray[3] myArray[4]

In the table of operator precedence in Chapter 2, “Working with Data,” you might remember that the two operators at the top of the table were the period and a set of square brackets. Each of those was referred to as being a “Member” operator. Hopefully now you can see why. One member operator is the pair of square brackets, [], and it is used to access the indexes of an array. The other member operator is the period (the full stop), and it is used to describe properties of an object like this: myObject.color myObject.size myObject.price

The period tells JavaScript to consider its right-hand operand as belonging to its left-hand operand. So document.title

tells JavaScript that the title property belongs to the document object. The square brackets mean exactly the same thing. Similarly, when we write myArray[2]

JavaScript is referred to the piece of data belonging to the array object myArray that has the index 2.

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By now you may be wondering why we would want to refer to the pieces of data belonging to an object by number. Well, one of the benefits of doing so is that you can use loop statements to loop through all the indexes of the array to perform the same process on each piece of data. Looping through the indexes of an array is possible because the indexes of an array are ordered! For example, myArray[2] comes immediately before myArray[3] which comes immediately before myArray[4]. The fact that array indexes are ordered makes looping and other uses of arrays possible. As you will see in the examples to follow, the numbering of an array starts from zero. Thus the first index in an array is written as myArray[0]

not, as you might expect: myArray[1]

Caution

Arrays are numbered from zero. Forgetting that can cause errors because if you refer to myArray[1] thinking it is the first member of an array (when it is the second), the result you will get is not likely to be the result you want.

Arrays are primarily used to structure data in JavaScript. By grouping pieces of data together under one object name, the data can be structured in a way that makes it much easier to use. In addition to having the ability to refer to each piece of data by number and therefore have the capability to use loop statements to get at the data, the Array object also has some very powerful methods for data management.

Note

If you find the concept of arrays difficult to grasp, then you might try thinking of them as a single column table or database. The column has a name (the array name), and each of the cells in the column is referred to by row number (the array’s index numbers).

Okay, enough theory; let’s actually make and use some arrays.

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Creating an Array Usually arrays are created using one of JavaScript’s Core Objects which, as you may have realized from Chapter 4, “JavaScript Is Object-Based,” is the Array object. This object also is referred to as one of JavaScript’s constructor functions, so called because they can be used to construct new objects.

NEW TERM

Caution

The first letter of the name of an Array object is capitalized, as is the case with all the other Core JavaScript objects. A common error is forgetting to capitalize the first letter. That simple mistake can cause some puzzling errors in your code.

When you use a constructor function to create a new object it is used in conjunction with the new keyword, as you may remember from the end of Chapter 4. The new operator acts on the Array() function (as it does when used with any other constructor function). It ensures that the function returns an object and not just a piece of data (some of the constructor functions always return an object, but some can return a piece of data or an object). Here is an example of how you would create an array using this method: var myArray = new Array();

The first thing you may notice about this example is that it uses the var statement to create a variable, called myArray, and then assigns it the new Array object. This is because arrays are stored in variables—or at least a reference is stored in the variable so that it looks as if the array is stored in the variable. Arrays don’t actually have a name of their own. They can only be accessed using data containers that store a reference to them. If that doesn’t make sense at the moment don’t worry about it. For now just think of the variable name as loosely being the name of the array. You will see how important the distinction is later.

Providing Individual Arguments Although the use of the constructor function above creates a new Array object, it has not been given any data to store in the array elements (which, put simply, is what the data containers are called that belong to an array and are accessed using index numbers). When the Array() constructor function is used to create an array there are two methods that can be used to create elements and store data in them. When only a small array is needed, it is common to pass each piece of data as an argument to the Array() constructor function. So, to create an array with three pieces of data “a”, “b”, and “c”, you would write: var myArray = new Array(“a”, “b”, “c”);

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When this method is used the function takes the data that is passed to it through the arguments and places each piece of data into the first elements of the array it creates. Note that the pieces of data are stored in the elements in the order that you write them. Therefore the array declaration above will create an array where the value of element 0 is “a”, the value of element 1 is “b” and the value of element 2 is “c”. Remember, the first element of an array is given the index 0, and the index of each subsequent element increases by 1.

Note

The reason arrays begin at index 0 is that computers (and therefore programmers) commence their counting from zero. Although this can initially take some time to become accustomed to, hopefully you will find it becomes second nature as you progress through the book.

Creating an Empty Array and Populating It Another method involves creating the array, and then populating (filling) its elements with data separately. To do this, you declare the array first, then on separate lines use the assignment operator to assign each piece of data to an array element one by one. The elements are written as you might expect. Just as you would use the property name of an object’s property to assign data to that property, like so: object.propertyName = “data”;

You can use an element’s index to assign data to array elements, like this: arrayName[index] = “data”;

So, if you rewrote the previous example using this second method for creating an array, you would write it like this: var myArray = new Array(); myArray[0] = “a”; // remember the first element index is 0! myArray[1] = “b”; myArray[2] = “c”;

Listing 5.1 creates an array that writes each of its elements back to the screen using a for loop.

INPUT

LISTING 5.1

Creating an Array, Populating It, and Displaying the Contents of Each Array Element (arrayContents.htm)



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LISTING 5.1

continued

Writing an Array

<script language=”javascript” type=”text/javascript”> ”); } //-->

Figure 5.1 shows the output when you run Listing 5.1.

OUTPUT FIGURE 5.1 Displaying the elements that are contained in a newly created array.

This method and the one shown earlier are simply different ways of doing the same thing. When only a small number of short pieces of data are needed to fill an array, the former is a quicker way to write it. But when you have a large number of longer pieces of data the first method becomes cumbersome and difficult to read. It is

ANALYSIS

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then better to use the second method just presented to individually assign each piece of data to an element after the array has been declared. For example, when you created an array earlier using var myArray = new Array()

the length of the new array was undefined. Sometimes you may want to create an array of a particular length, say 10 elements, at a time when you don’t necessarily know the data (or all of the data) with which the array is going to be populated. So, a new array of a particular length can be declared like this: var myArray = new Array(10);

The above code causes an array with 10 elements to be created, although each element is presently empty. The number in parentheses tells the function how many elements the array should be created with. It makes no perceivable difference to the speed with which you can create the array, nor does it prevent you from adding more elements later. When it comes in useful is if you need an array to be a certain size but don’t yet want to populate its elements. Remember that when the length is 10, the first element in the array is myArray[0], and the final element in the array is myArray[9].

Note

Just as you can assign and reassign different types of data to a variable, all of the array elements need not have the same data type. Unlike more fully featured programming languages, JavaScript allows you to change the data type of array elements.

Array Literals As mentioned at the end of Chapter 2, in addition to using constructor function syntax, you also can create copies of objects using a literal notation. To achieve the same result as in the examples shown above, you could use array literals to declare the array, like this: var myArray = [“a”, “b”, “c”];

This third way of declaring an array hasn’t been commonly used until recently because it was introduced with version 4 browsers. Now that the vast majority of browsers are version 4 and above, it is fairly safe to use this method unless you know that you must cater to earlier browser versions.

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When you use literal notation to create an array you do not need to use the Array() constructor function. Simply list the pieces of data in the order that you want to place them into the array’s elements, and then enclose this list within square brackets. The JavaScript interpreter knows that the square brackets are indicating that it should create a new array and use the list of data to fill the elements. When using literal notation to create an array, the array that is created is no different than the array that would have been created had the Array() constructor function been used. Therefore you can use any of the three methods without worrying about the differences. However, for the remainder of this chapter, let’s concentrate on using the constructor function notation because it is the only general way of guaranteeing—with other Core JavaScript objects—that the objects we want to create are actually objects. For arrays, using the constructor notation is one of several useful forms of syntax. For other objects, it will be essential for you to use the constructor function. So using the constructor function to create arrays will help you become accustomed to using the constructor function notation.

Accessing Arrays To use the data in the elements of the arrays shown above, simply reference them as you would when accessing a variable, object property, or any other data container. Just be careful to specify the index of the element whose data you want. For example, if you wanted to obtain the data from the second element in the examples above, you would write: // Remember the second element has the index 1! var myVal = myArray[1]; alert(myVal);// alerts “b”

Let’s look at something a bit more interesting, looping through the elements of an array. By structuring data in arrays, you can use loop statements to loop through the elements. Any time you have a repeated process where the only change is a piece of data, a combination of loop statement(s) and array(s) is ideal. In the following example (Listing 5.2), let’s write out some phone numbers to an HTML table. This could be done directly, but to demonstrate how we can combine arrays and loop statements let’s use JavaScript. Note that because you will be writing HTML in the body of the document, you will have to place the script block into the body of the page.

An Introduction to Arrays

INPUT

LISTING 5.2

169

Looping Through an Array (arrayLooping.htm)

Table of Phone Numbers

<script language=”javascript” type=”text/javascript”>
= = = = = =

new “31 “49 “44 “01 “02

Array(); 20 305355”; 30 20670 0”; 020 4562 2929”; 518 463 5622”; 9663 0551”;

for (var i=0; i<5; i++) { document.write(“”); document.write(“”); } //-->

Phone Number “ + i + “ is:

” + phoneNos[i] + “

As the for statement loops through the array plucking data from each element and writing it to the page into the table’s rows and cells, the table is populated, and when the page loads you should see something like Figure 5.2.

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OUTPUT FIGURE 5.2 Tabulated phone numbers.

As you can see, even before you begin the script block you have already written the opening tag of the table. It shouldn’t be included in the script block because it is only written once. However, the process used to produce each row needs to be repeated, and so fits well within the functionality of a for loop. So to recap, the first thing you have done in your script is to store the phone numbers in an array called phoneNos. You then set up a loop statement. In this example, we used a for loop. The first thing you need to think about when designing the loop is what the three statements will consist of (the three statements are contained within the parentheses of the for statement). The index of the first element of the array is 0. So you will need to set the initial value of the counter variable i to 0. As there are five pieces of data in the array, the index of the array’s last element is 4. Therefore set the condition of the array so that it will continue to loop while i is less than 5, which means that 4 will be the last value of i that will be processed by the for loop. Lastly, of course, set i to be incremented because you will want to iterate through the elements. Now let’s look at the statement block. The statements that the for loop governs clearly need to contain the document object’s method if the contents of our array elements are to be written to the page. It is the code contained within the parentheses of the write() methods that may be a little more tricky to devise. write()

In the first line of the statement block, start a new table row and write out the first cell. Concatenate the string “Phone Number “ with i and another string “is:”. With each loop the value of i will be different, so you will end up with a different string in the first cell of each row of the table with the format: Phone Number X is:

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where X is the value of i during the loop that wrote the row in which the cell is contained. In the second line of the statement block, write out the table cell that will hold the appropriate phone number. Of course this time you will need to extract a phone number from the array phoneNos. The clever thing here is that you do not specify an index in the square brackets. Instead you use the counter variable i. Since i is the looping variable the index for each loop is different. On the first loop the value of i is zero, and so the phone number stored in phoneNos’s first element, phoneNos[0], will be written to the page. The second time through the value of i is 1 and the contents of phoneNos’s second element, phoneNos[1], will be written to the page. The looping continues until the data contained in all of the elements of the phoneNos array has been written to the HTML page. Although in this example it would have been just as quick to write out the table as HTML without the use of JavaScript, using arrays can often save space. The Array object’s powerful methods also allow you to organize and change this data before it is outputted. Before looking at the Array object’s properties and methods, let’s first look at using arrays in parallel.

Parallel Arrays There are times when you may want to work with two or more related sets of data. The most straightforward way to do this is to structure the data in parallel arrays. In this instance, parallel means that the sets of data are stored in separate arrays, but an element of any given index in one array is related to the element(s) at the same index in another array(s). To illustrate this let’s look at an example. Imagine that the phone numbers in the previous example are the numbers of our international offices. If you want to output the phone numbers into an HTML table as you did in the previous example, you probably will want to write them alongside the office locations. To do this you will need to store the office locations in an array as well as the phone numbers. The key to success is making sure that the two arrays are parallel. But what does this mean in practice? Let’s assume that the array where the locations will be stored is called “offices”. If the first of our phone numbers is the number of our Amsterdam office, then Amsterdam needs to be written into the table in the element with the same index as the first phone number. For this to happen, “Amsterdam” needs to be stored in the element of the array offices with the index that corresponds to the index of the first phone number. In this case the

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element index is 0. This means that in the loop when the counter variable i’s value is 0 and the first phone number is written, “Amsterdam” will also be written in the same loop and therefore will be in the same table row. For the same reason if the second number (stored in the element of phoneNos with the index 1) is the number of the Berlin office, then “Berlin” needs to be stored in the array offices with the index 1. Given that the next three telephone numbers are the numbers for the London, New York and Sydney offices respectively, the logical progression is shown in Table 5.1. TABLE 5.1

Tabulation of offices and phoneNos Arrays

Element Index

Office

Phone Numbers

0

Amsterdam

31 20 305355

1

Berlin

49 30 20670 0

2

London

44 020 4562 2929

3

New York

01 518 463 5622

4

Sydney

02 9663 0551

Let’s implement this in a new HTML page. See Listing 5.3.

INPUT

LISTING 5.3 Creating the Parallel Arrays—offices and phoneNos (parallelArrays.htm)

Office Phone Numbers

<script language=”javascript” type=”text/javascript”>
= = = = = =

new Array(); “Amsterdam”; “Berlin”; “London”; “New York”; “Sydney”;

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LISTING 5.3

173

continued

var phoneNos phoneNos[0] phoneNos[1] phoneNos[2] phoneNos[3] phoneNos[4]

= = = = = =

new “31 “49 “44 “01 “02

Array(); 20 305355”; 30 20670 0”; 020 4562 2929”; 518 463 5622”; 9663 0551”;

for (var i=0; i<5; i++) { document.write(“”); document.write(“”); } //-->

” + offices[i] + “

” + phoneNos[i] + “

This script will write out the table and when the document is loaded, you should see the following page in your browser window. See Figure 5.3.

OUTPUT FIGURE 5.3 International office locations with their telephone numbers.

Of course you aren’t limited to just two arrays in parallel. You could also have an array for street addresses, an array for e-mail addresses, and so on. It is important to remember that the elements must line up so that the arrays are “parallel.” Okay, now that you know how to create and use arrays, let’s find out more about the properties and methods that the Array object makes available to you when working with arrays.

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Array Properties The Array object has five properties. These have been listed below: • The constructor property • The index property • The input property • The length property • The prototype property This chapter will look at only one of the properties of the Array object in detail—the property. Although what it does is very simple, it can be an extremely useful tool. Often when you use a loop statement to access an array you will use the length property. Let’s see how it works so that you can understand why it is so useful. length

The length property simply tells how many elements an array contains. This is demonstrated in Listing 5.4. LISTING 5.4

INPUT

Using the length Property of the Array Object (arrayLength.htm)

<script language=”javascript” type=”text/javascript”>

Array Length

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If you were to run this piece of script you would see the following alert box. See Figure 5.4.

OUTPUT FIGURE 5.4 Using the length property of the Array object to display the length of the array lengthTest.

Note that even though the last element in the array has an index of 2, the alert shows the length property has the value 3. This is because the indexing of arrays starts at 0—but there are still three elements. This is a feature that can be confusing initially, so be careful that you don’t trip up on that.

ANALYSIS

The index of the last element of an array is always one less than the value stored in the array’s length property. So if you were to use the length property to retrieve the value of the last element, you would have to subtract one from it as shown in the following example: var valueOfLastElement = myArray[myArray.length-1];

In the table of operator precedence, the square brackets were at the very top of the table, which means that you can put expressions inside square brackets. The content of the square brackets will evaluate to a single value before the rest of the statement is evaluated. In the example above the expression myArray.length returns the number of elements in the array, and then 1 is taken off so that the result is the index of the last element in the array. Therefore the contents of the last element of the array are assigned to the variable called valueOfLastElement. When you need to add a new element to the end of an array, you also can make use of the length property. This time, however, let’s use the length property unaltered: myArray[myArray.length] = “value for new last element”;

Again the value of the length property is returned before the rest of the statement is evaluated. The length of the array is one greater than the index of the last element of the array because the first array index is zero. Therefore the index of myArray, which is one past its last element, is assigned the string you see above. This effectively creates a new element at the end of the array. Note also that after the statement has been evaluated the

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length of the array is increased to reflect the addition of a new element. Therefore you could use this statement in a loop to repeatedly add new elements to the end of an array. Listing 5.5 shows the length of an array before and after new elements have been added in the manner just discussed.

INPUT

LISTING 5.5

Using the length Property of the Array Object to Add Elements to the End of an Array (lengthUpdated.htm)

<script language=”javascript” type=”text/javascript”>

Length Automatically Updates

ANALYSIS

When you load this page you will see three alert boxes in succession. These alert boxes will contain the values 1, 2, and 3 respectively.

Both these examples are good uses of the length property, but where the length property of the Array object can be most helpful is in looping statements. To demonstrate that let’s look at another example. In the two examples earlier with the telephone numbers and office locations, you had to specify in the for loop statement exactly how many elements were in the array. If you had used a number that was too large, the loop would have looped too many times, but if you had used a number that was too small not all of the office locations and phone numbers would have been written into the table. Rather than counting the number of elements

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in the loop you could have used the array’s length property. Let’s see how to create the page again with this modification. See Listing 5.6.

INPUT

LISTING 5.6

Using the length Property to Control How Many Times a for Loop Loops (lengthDemo.htm)

Office Phone Numbers

<script language=”javascript” type=”text/javascript”>
= = = = = =

new Array(); “Amsterdam”; “Berlin”; “London”; “New York”; “Sydney”;

= = = = = =

new “31 “49 “44 “01 “02

Array(); 20 305355”; 30 20670 0”; 020 4562 2929”; 518 463 5622”; 9663 0551”;

for (var i=0; i”); document.write(“”); } //-->

” + offices[i] + “

” + phoneNos[i] + “

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In the parentheses of the for loop we have told it to keep looping while i is less than the length property of (number of elements in) the array offices. This means that the last loop that will be made is with i set to one less than the length of the array. This will always be the index of the last element of an array, which is exactly what we want.

ANALYSIS

Note

In this example we have specified the length property of the array called offices, but we could just as easily have used the phoneNos array because both have the same length.

For a short simple script like the one above, using the length property or writing in the length doesn’t really make much difference. Either method is almost as easy as the other. However, for many of our longer or more complex scripts this won’t be the case. Often writing the length of an array into loop statements as a number can be inconvenient or may even be impossible, since you can’t necessarily know in advance the length of a particular array. When you have an array that changes regularly and you have hardcoded its length into loop statements as a number, you will need to change these numbers every time you change the array. Tracking down hardcoded lengths in loop statements at various parts of a script can be a real pain—especially in longer more complex scripts. And, of course, if you accidentally miss making one correction or update your script, you will very likely produce all sorts of unexpected and hard to track down errors. You will also come across occasions when you don’t know how many elements an array will contain. This can be because certain processes will alter the length of your arrays by an unknown amount, the arrays are built up from a server-side database, or your site. Regardless of the reason why you can’t predict the length of the array, using the length property then becomes the practical way to write your code.

Tip

The length property should always be used in the condition of the loops that are created to handle arrays. Getting into that habit will save both time and effort in the long run, even if it makes the for statement just a little longer.

Having already looked at the length property, let’s briefly touch on the other properties of the Array object, which we will look at in more detail in later chapters.

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There are two properties that the Array object has that all JavaScript Core objects have in common. These are the prototype and constructor properties. Both of these properties are intended to enable you to do some advanced scripting, as you will see later in the book. (We will not continue to mention them as we look at each of the other Core objects, but you may wish to be aware that they exist.) Another two properties of the Array object, index and input properties, are specific to the Array object and are sometimes generated by regular expressions. Regular expressions are a fairly advanced topic that is discussed later in the book.

Array Methods What the Array object lacks in terms of number of properties it more than makes up for with the number of its methods. The methods that belong to JavaScript’s Array object provide us with powerful tools that make managing arrays and the data they contain much easier. Here is a list of the methods of the Array object: • The concat() method • The join() method • The pop() method • The push() method • The reverse() method • The slice() method • The shift() method • The sort() method • The splice() method • The toLocaleString() method • The toSource() method • The toString() method • The unshift() method • The valueOf() method In this section we will examine most of these methods, but will leave the concat(), and splice() methods until later in the book. Although those are the most powerful of the Array object’s methods, they are also the most complicated to use, so for now let’s satisfy ourselves with the others. slice(), sort(),

First, let’s look at the first three methods, which return the contents of the array they are applied to as a string. There are occasions when this can be useful, but not often, so we

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won’t spend too much time on them. The other methods that we will look at in this chapter provide a means of organizing and using the data in our arrays, and as such are worthy of careful attention.

The toString() Method When you create an array and pass it by name as an argument to an alert() function you see a list of the values in each element separated by commas. For example, as shown in the following:

INPUT

var x = new Array(1,2,3); alert(x);

This brings up the alert box seen in Figure 5.5.

OUTPUT FIGURE 5.5 The content of a new array shown in an alert box.

ANALYSIS As you can see the contents appear as a comma-separated string, but in reality it only exists in this form in the alert box—the array is still an object with all the data in individual elements.

If you want to obtain a string list of the elements of an array as they appear in the alert box, then you need to use the toString() method. As a global function that is applied using the dot notation, the toString() function is also available to all objects (including the Array object) as a method. When used on an array the toString() function creates a string from the elements in the array separated by commas—exactly as you saw them in the alert box. It isn’t common to use the toString() method, but if, for example, you need to send data you have collected and stored in an array to the server (or another page), then it can make things a lot simpler to send the data as a comma-separated list. Later in the book we will see how to pass data between pages, but for now let’s continue with our study of the Array object’s methods.

The toLocaleString() Method The toLocaleString() method has been introduced to JavaScript only recently and is supported by only Internet Explorer 5.5+ and Netscape 6+. It too returns the contents of

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all the elements of an array strung together as a string, but with a fundamental difference to the toString() method. The toLocaleString() method returns a string that contains all the numerical elements of the array in local currency format, leaving other data types unchanged, and so creates a single string from all the array’s elements modified to local format. As is the case for the toString() method, the toLocaleString() method is available as a method of all the JavaScript Core objects. As formatting numbers is more appropriate for discussion in Chapter 7, “Numbers and Math,” let’s leave a more detailed examination of this method until then. For the moment just note for the future that the toLocaleString() method can be applied to arrays of numbers, as well as the numbers we will see it applied to in Chapter 7.

The join() Method The join() method is the third method for returning the contents of array elements that are “joined” together as a string. Used just as it is, without a parameter, the join() method will behave in exactly the same fashion as the toString() method. The join() method will simply return a string of the elements of the array that it’s applied to separated by commas. But when you pass it a value as a parameter, you can control the separator used. With the toString() method, the items contained within the elements of an array are always returned separated by commas. Sometimes you may wish or even need to separate the elements with some other character (or series of characters). By passing these character(s) to the join() method as its sole argument, you can do exactly that. For example:

INPUT

var myArr = new Array(1,2,3); var myStr = myArr.join(“---”); alert(myStr);

This will bring up the alert box seen in Figure 5.6.

OUTPUT FIGURE 5.6 Displaying the contents of an array using the join() method.

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ANALYSIS

As you can see each element has been separated by the three dashes passed to the join() method as its argument.

Of course this method can also be used for passing array data to the server or another page, and you may choose it over the toString() method if there is a reason for separating the elements with characters other than commas. But, perhaps surprisingly, the join() method also comes in very useful when you need to join the elements of an array without a character separator. To do this, simply pass a zero length string as the method’s parameter. To do this for the previous example, you would have written it like this: var myStr = myArr.join(“”);

For the moment we won’t make much use of this method, but if you need to build up parallel sets of data in small steps, a combination of arrays and the join() method is invaluable.

Adding and Removing Elements from an Array Netscape has had five methods for adding and removing elements from an array since version 4. Two of these methods add and remove elements to and from the beginning of the array, and the other two methods perform the same function at the end of the array. The fifth method is used to add, remove, and edit elements at any point in an array; it is the splice() method, which is one of the methods we will look at later in the book.

Caution

Unfortunately these methods were not introduced to Internet Explorer until version 5.5. In the future when the version 4 and 5.0 browsers have virtually disappeared, you will be able to make full use of these methods. Note that Internet Explorer 5.0 and under won’t recognize them.

Let’s look at the first two methods that work at the end of an array. Part of the work done by these two methods can be (almost) as easily achieved by other means, but they also have some helpful features that add to their usefulness.

The push() Method You can make use of the length property (as discussed earlier) when you want to add an item to the end of an array. For example, to append a new element to an array called myArray you could simply write: myArray[myArray.length] = “data”;

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The value contained in myArray’s length property will be one greater than the index of myArray’s last element. Therefore when the statement between the square brackets evaluates the string “data” is added to a new last element at the end of the array. After this line of code is executed the length property is automatically updated to reflect the change. That is fairly concise but you also can use the push() method to achieve the same result. Use the dot notation to apply the method to an array, while supplying the data for the new element as a parameter. If you were to rewrite the previous example to make use of the push() method, it would look like this: myArray.push(“data”);

This is slightly shorter and slightly more readable—especially when the name of the array is quite long. It also can be used to populate an array as shown in the following example: var countArray = new Array(); for (i=0; i<5; i++) { countArray.push(i); } alert(countArray); // alerts “0,1,2,3,4”

However there is a little trick that the push() method has that the above examples don’t show. The method also returns the new length of the array it has been applied to, as shown in Listing 5.7:

INPUT

LISTING 5.7

The New Length of an Array Is Returned by the push() Method (lengthReturned.htm)

<script language=”javascript” type=”text/javascript”>

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LISTING 5.7

continued

New Length Returned

ANALYSIS

The second line not only assigns the value 4 to the end of the array x, but it also returns the new length of x which has been assigned to the variable newLength.

The pop() Method The pop() method is similar to the push() method in that it works at the end of an array, but exactly opposite in terms of the action it performs. The pop() method removes the last element from the array it is applied to as you can see in Listing 5.8:

INPUT

LISTING 5.8

Using the pop() Method to Return the Last Element of an Array (lastElement.htm)

<script language=”javascript” type=”text/javascript”>

Last Element Returned

A loop statement also can be used in conjunction with the pop() method to loop through an array, removing all its elements one at a time and performing an action on each one as it is removed.

ANALYSIS

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Let’s look at an example that uses a combination of the pop() and push() methods together with a while loop to separate an array of ages into ages belonging to adults and ages belonging to minors. For the purposes of this example, let’s assume that a minor is less than 21 years of age. See Listing 5.9. LISTING 5.9

INPUT

Using the pop() and push() Methods of the Array Object to Separate Ages (agesSeparated.htm)

<script language=”javascript” type=”text/javascript”>
adults.length + “ adults “; “ + adults.join(“, “) + “
”; minors.length + “ minors “; “ + minors.join(“, “);

//-->

Separating an Array

<script language=”javascript” type=”text/javascript”> ” + msg + “

”); //-->

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This will generate an appearance on a page similar to that shown in Figure 5.7.

OUTPUT FIGURE 5.7 Ages separated and recombined using the pop() and push() methods of the Array object.

The first three lines of code create three arrays. The first one (ages) has six numbers in it, representing a group of people’s ages, and the other two arrays (called adults and minors) start out empty. Let’s remove all the ages one by one from the array ages and move them to one of the other arrays depending on the value of the array elements. In this way the ages of the minors can be separated out from the ages of the adults.

ANALYSIS

Note that by using the length of the array called ages, you don’t even need to use a loop counter or incrementer for your loop. Simply use a while loop to keep removing and sorting elements until the length of the array ages is zero. Once this has happened (when all the elements have been pulled off and sorted), then the loop will stop. In the statement block of the while loop, first remove the last element from the array called ages and store the value returned in a variable called tempAge. Then use if...else statements to choose to which array to add the age that was removed. The loop continues until all the ages have been assigned to the correct arrays. After the while loop a message is built up to show the contents of the two arrays: and minors. In the first line of this code, you will find the number of adult ages that have been found by using the length property of the adults array. In the next line, the join() method is used to join up these ages to display individual ages separated by a comma and space. After adding a
tag, proceed to do the same with the minors array. In the body of the document, we finally output the separated array in a paragraph. adults

You may have noticed that the order of the individual ages was reversed by the method used in Listing 5.9. If we had used the reverse() method within the code, to be described later, we could have maintained the ordering of the data. If we had other arrays that were parallel with the array called ages, such as an array for names or contact details, we could also have separated them as well. We would just have had to create two new arrays for each original one.

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The shift() and unshift() Methods The names of the methods used to add and remove elements from the front of an array are not quite so vivid as the names push() and pop(). The shift() method is the method that does the same thing to the beginning of an array as the pop() method does to the end. It removes and returns the first element of the array to which it is applied. For example, see Listing 5.10.

INPUT

LISTING 5.10

Using the shift()Method to Remove the First Element (firstElement.htm)

<script language=”javascript” type=”text/javascript”>

Removing the first Element

There are several things to note about this example. The first alert tells us that the shift() method has removed the first element as we said it would, and the second alert shows that the length property of array x has been reduced to 3. But it is the third alert that is really important to note. It shows that the data in the rest of x’s elements have all been moved back by 1. If this was not the case, then the first element would have been left empty, and the next time we tried to use the shift() method it would simply have returned the value undefined.

ANALYSIS

The unshift() method, as you may have guessed, is the method to add a new piece of data to the beginning of an array. Listing 5.11 shows an example of the unshift() method at work.

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INPUT

LISTING 5.11

Using the unshift() Method to Add an Element to the Beginning of an Array (unshift.htm)

<script language=”javascript” type=”text/javascript”>

Adding to the Beginning

ANALYSIS As you can see above, it has pushed the number 0 onto the front of the array x

and moved all the other elements back by one. Note that just as the push() method returns the new length of the array, so does the unshift() method—but only in Netscape. For some reason at the time of writing, it is not returned in version 5.5 of Internet Explorer. The shift() and unshift() methods are just an alternative way to add and remove elements to and from an array. Sometimes you may want to add to or remove elements from the end of an array, and sometimes you may want to add to or remove elements from the beginning of an array. The Array object provides methods to do both.

If you need to add or remove elements in the middle of an array, you will need to use the splice() method. However, it is one of the four methods of the Array object that we will leave until later in the book to examine in detail.

Caution

Remember that the push(), pop(), shift(), and unshift() methods were only introduced to Internet Explorer at version 5.5. For most of us, this means that we will have to wait some time before we can safely use these methods in a production Web site.

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The reverse() Method As its name suggests, the reverse() method reverses the order of the array elements. Once applied to an array the element that had index 0 will now have the index of the last element in the array. The element that was last will now be first, and all the elements in the middle are moved to their corresponding mirror-image position. This very fast and efficient means of reverse ordering an array provided by the reverse() method can be very useful. For example if you knew an array was in alphabetical order and wanted to put it in reverse alphabetical order, then you would simply use the reverse() method. In Listing 5.9 we used the push() and pop() methods to split an array of ages into adults and minors. If we add the reverse() method prior to separating the array in that listing, as shown in Listing 5.12, we would preserve the original ordering of the ages.

INPUT

LISTING 5.12 Using the pop(), push(), and reverse() Methods to Split an Array While Preserving the Order of Elements (orderPreserved.htm)

<script language=”javascript” type=”text/javascript”>

adults.length + “ adults “; “ + adults.join(“, “) + “
”; minors.length + “ minors “; “ + minors.join(“, “);

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LISTING 5.12

continued

Separating an Array

<script language=”javascript” type=”text/javascript”> ” + msg + “

”); //-->

As you can see in Figure 5.8, the ordering of the ages which existed in the ages array has been restored by adding the reverse() method to the code created earlier.

OUTPUT FIGURE 5.8 Using the reverse() method to preserve the ordering of ages in the new arrays.

A very useful application of the reverse() method is as a workaround for the lack of support for the shift() and unshift() methods in Internet Explorer 5.0-. The main task of the push() and pop() methods can be easily replicated by using the length attribute to either assign or remove elements from the end of an array. However, the shift() and unshift() methods are not so easily replicated. Unless of course, we use the reverse() method to reverse the array so that we can use the length property to add and remove elements in the same way as we would use it as an alternative to the push() and pop() methods. Then we can reverse the array back to its original order, and it will be as if we had added or removed the elements from the front of the array. For example, assuming we start with an array with the values 2, 3, and 4 in its first, second, and third elements, respectively, and want to insert the values 0 and 1 at the beginning, we might use code similar to that shown in Listing 5.13.

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INPUT

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LISTING 5.13 A Technique Using the reverse() Method to Emulate the unshift() Method for Adding Elements to the Beginning of an Array (addElements.htm)

<script language=”javascript” type=”text/javascript”>

Adding Elements

ANALYSIS By doing this you effectively add the elements to the start of the array. This saves you from having to write significantly more complex scripting in order to move all the elements up by one each time you need to insert a new first element.

Caution

When adding more than one element to a reversed array you must add the innermost element first, progressing to what will ultimately be the first element of the array.

You can also use the reverse() method when you want to remove elements from the beginning of an array. This time assume an array starts with a complete list of numbers from 0 to 4 in its corresponding elements and you want to remove the first two elements completely.

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LISTING 5.14

INPUT

Using the reverse() Method to Emulate the Effect of the shift() Method to Remove Elements from the Beginning of an Array (removeElements.htm)

<script language=”javascript” type=”text/javascript”>

Removing Elements

This time for each of the two elements we have removed, we have first read their data into a variable (presumably for some sort of processing), and then reduced the length of the array by one to remove that element. Although not as simple as the shift() method, by using the reverse() method in this way we are saved from the trouble of constructing scripts to shift element data back, element by element, before finally removing the last element of the array.

ANALYSIS

Let’s move on to look at two of the methods listed earlier that you are less likely to use. One of these is the toSource() method and the other is the valueOf() method.

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The toSource() Method When applied to an array the toSource() method returns the array’s contents as a string in the form that might have been written in the code to create the array in the first place. This is best explained using an example, so take a look at the following piece of code: var x = new Array(1,”b”,true); var y = x.toSource(); alert(y); // alerts [1, “b”, true]

As you can see the string contained in the variable y looks the same as the literal notation that could have been used to create the array x. Not only have the elements been separated with commas (and optional spaces), but the square brackets are also in place. The theory is that this string could be turned into code again using the eval() function to create an identical copy of the original array. There are times when this could be useful, but unfortunately the toSource() method is supported only by Netscape. Unless you know that you will only be scripting for Netscape users, you would be wise not to use the toSource() method.

The valueOf() Method The valueOf() method is of even less use. In fact, in the context of arrays, it is of no use at all. It exists as a method of the Array object simply because the Array object inherited it from the object that created it, the Function object (remember what we said in the previous chapter about all the Core objects being created from the Function object). When used on an array the valueOf() method simply returns the array itself—it is the same as if we hadn’t included the method at all. However, that is not to say the valueOf() method is useless in the context of other objects! You will see later in the book that it can indeed sometimes be useful. It just means that in the context of arrays the method can be safely ignored.

Associative Arrays Throughout this chapter we have focused on the benefits of storing data in data containers that we can access using index values instead of names. This is, and will continue to be, our primary way of storing data in arrays. However there is a second method that is available which, surprisingly enough, involves creating array elements using strings rather than index values. For example, after defining the array x, you can create elements like this: var x = new Array(); x[“first”] = “1st”; x[“second”] = “2nd”; x[“third”] = “3rd”;

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This type of array is commonly known as an associative array. This is because when elements are created in this way the data they contain is associated with, and can be accessed using, the string in the square brackets.

NEW TERM

Associative arrays are not used very often. In past versions of JavaScript, array elements could be created and accessed using both the string they were associated with and an index value. This is no longer the case (at least not for the arrays that we create). However, associative arrays can be used to teach us something more about the nature of objects and how we access them. The data containers that are created when an element is declared using a string instead of an index value are not included in the indexed elements of the array. They cannot be accessed using an index value, and they do not affect the length property (and so we can’t cycle through them using loop statements). The reason for this is that they are not actually elements of the array, they are properties. To demonstrate, try Listing 5.15.

INPUT

LISTING 5.15 Creating and Displaying an Associative Array Using Two Notations (notations.htm)

<script language=”javascript” type=”text/javascript”>

Mixing Notations

This example will bring up the following alert box, shown in Figure 5.9.

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OUTPUT FIGURE 5.9 Displaying the contents of the associative array in an alert box.

Despite creating the data containers using square brackets and a string, you have been able to access these “elements” (actually properties) by using the dot notation. What this teaches us is that in fact the dot notation and the notation where we use string values in square brackets are just two different ways of accessing the same thing. Both were marked as “Member” operators in the table of operator precedence in Chapter 2, “Working with Data,” and as such both signify that the name/string they operate on respectively are members of the preceding object. All JavaScript objects can be accessed using either method. For example: window[“document”]

is equivalent to window.document

and window.document.body

is equivalent to window.document[“body”]

In fact, we can even write a series of object and property names as strings in square brackets: window[“document”][“body”]

Try it for yourself by running any of the following lines of code: alert(window[“document”]) alert(window.document[“body”]) alert(window[“document”][“forms”])

All of these alert functions will alert you to the fact that you have reached an object. The only thing you cannot do is begin the first object in your object path in this way. For example: [“window”][“document”]

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This will cause an error because you cannot begin a line of code with an opening square bracket in JavaScript. Doing so assumes you are starting a new array using literal notation. The JavaScript interpreter hasn’t yet come to the later set of square brackets so it doesn’t know it is part of a path name. In the above example, it would start at the left and create a single element array where the first element had the string value “window”. When it then came to the second set of square brackets with the string “document”, it wouldn’t know what to do with them because you can’t write two sets of array literals one after another. Hence an error would result. Clearly, there is no real use for accessing objects, properties, or methods using square brackets containing their names as a string, in open code. But when you pass object names to a function for example, you may pass the object’s name as a string rather than as a reference to the object itself. In this case with the name already in a string value, the simplest way to access the object is to insert the function argument that contains the object name into a set of square brackets in the object path. This saves you from the more awkward use of the eval() function. An example of when the square bracket notation must be used was shown in the example in Chapter 4 when we used a for in loop to explore an object called “bike”. Hopefully you will now be able to understand the syntax we used to return the value of each of bike’s properties. Just to make sure that you fully understand what we did earlier, let’s go over it. Here is the code again. var props = “”; for (var prop in bike) { props += “The bike’s “ + prop + “ is “ + bike[prop] + “\n”; } alert(props);

The property name that is given to the variable prop during each loop is in the form of a string. Therefore to find out its value you cannot simply write bike.prop. You must access the property value by enclosing the variable prop in square brackets to show that the string belongs to the object bike. Note that, conversely, indexed elements can be created for any JavaScript object, not just instances of the Array object. The difference is that the Array object is specifically designed for working with indexed elements. Only with instances of the Array object do you get access to its length property and a host of useful methods—the most powerful of which we haven’t discussed yet. In general, in JavaScript if you want to create a data structure where you can access the data using object and property names, you use the Object object; but, if you want to order, sort, and iterate through your data, you use an Array object. We will see how to use the Object object later in the book.

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Storing Objects in Array Elements Arrays can store references to objects in their elements as well as simple data (literals). Perhaps the best examples of this are the collections belonging to the Document object that we briefly looked at in the previous chapter. These collections are in reality simply arrays containing elements that hold shortcut references to other objects as opposed to data. There are three ways of using these arrays. To demonstrate, let’s assume we want to access a form with the name “myForm” that is the second form on a page. We could access it using any of the following three lines: document.forms[1] document.forms[“myForm”] document.forms.myForm

When we know the order in which an object comes in relative to the other objects in a collection, then we can use an index value to access it (as shown in the first example. Remember that the second element of an array has the index 1!). In addition to this, as long as the object also has a name, then you can also access it using either its name as a string in square brackets (if necessary), or more frequently by referring to it with the dot notation. These two methods of accessing a form are shown in the second and third lines respectively. An HTML collection is the only object where we are able to automatically access its child objects using their names as well as an index number. Although the forms collection isn’t the best example, for some collections it is essential to be able to loop through all its objects as an array. An example of this is the options collection belonging to the forms collection, which we will meet in Chapter 6, “HTML Forms and the String Object.” If it is possible to access an object by name, then that is probably the better of the two methods. Referring to objects by name provides flexibility to reorder your Web pages with ease. If you refer to objects by index in your scripts, then you may need to search through your scripts to change the index numbers used to reference elements when you change the page. But if the references are by name, then you can move things about without worrying about the effects of reordering on your scripts. Listing 5.16 provides a further example where the links collection is used to add a query string to all links on a page.

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INPUT

LISTING 5.16

Modifying the Links Collection (modifyLinks.htm)

Link Modifier

Example
Example
Example
Example
Example
Example


/> /> /> /> /> />

<script language=”javascript” type=”text/javascript”>

In this example, we placed the script block after the contents of the body. This is because if our script tried to change the links on the page before they had arrived in the browser, then it would result in an error.

ANALYSIS

In the first line of our script, we build up a query string. We have simply concatenated a question mark to another string, but we could have used other processes to build up the query string, such as extracting user data from a cookie so it can be passed to the server depending on which link the user clicks. In the second line of our code, we assign a reference to the links collection to a variable called allLinks. This saves us from writing document.links in our for loop, helping to keep the code more concise and easier to read.

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In the parentheses of the for loop, we set the conditions so that it will loop through all the link objects in the links collection. We then assign the href property of each link with the href property concatenated to our query string. This has the effect of simply adding the query string to the link. If we hadn’t concatenated the URI in the href property with the query string to reassign to the href property, then the query string would simply overwrite the link.

Summary In this chapter we have looked at arrays. As we progress through the book their power and usefulness will become increasingly apparent. In Chapter 14, “Advanced Array Management,” we will return to the subject of arrays and look at some more advanced topics. Always look out for ways of using arrays to structure your data so that your scripts can run efficiently. Whenever you have a script that performs a repeated operation on some data, ask yourself if this data might be best organized as an array so a loop statement could be used to make the process easier. This can be much more efficient than hardcoding multiple lines of code. The next chapter will explore the Form object and its child objects. To use the Form object you will need several of the skills discussed in this chapter for array management.

Workshop In this workshop we will review what you have learned about arrays in this chapter.

Q&A Q. Is there an easy way to order the data I store in my arrays based on certain criteria, or do we have to write our own scripts to do this? A. The four methods of the Array object that we didn’t look at in this chapter are very powerful tools. One especially, the sort() method, is designed specifically for the job of rearranging the data stored in an array based on criteria we specify. We will be taking a detailed look in Chapter 14, but if you need the capabilities it provides then feel free to skip to Chapter 14. (You should have learned enough by now to be able to understand the description of how the sort() method works.)

Quiz 1. What is the index number of the third element in an array? 2. What are the methods of the Array object called that remove an element from the end of an array and add an element to the end of an array?

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Quiz Answers 1. The index number of the third element in an array is two. The first element in an array has an index of zero; therefore, the second element has an index of one, and the third element has an index of two. 2. The pop() method removes an element from the end of an array. The push() method adds an element to the end of an array.

Exercise Make up some names and create an array called names in parallel with the array called ages that we separated earlier. Once you have done this add some code to the for loop that will separate out the names with the relevant ages. Tip: you will need to create another two arrays perhaps called minorNames and adultNames that the separated names should be added to. These two arrays should end up in parallel with the arrays minors and adults respectively.

WEEK 1

DAY

6

HTML Forms and the String Object User interaction with scripts can come in many forms (forgive the pun), but the primary means of collecting input from a visitor to a Web site remains the humble HTML/XHTML form. Forms provide ready-made HTML/XHTML elements that allow users to interact with a Web page (for example, by entering text, selecting options from select boxes, and ticking checkboxes). So understanding how to manipulate HTML forms using JavaScript is a particularly important subject if you want to efficiently gather information from visitors to your Web site. Since much of the data collected using HTML forms is in the form of strings, we also will introduce you to techniques that will help you handle strings, which a user may type into a text field or textbox.

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This chapter will teach you • Form properties and methods and how to use some of them • How to collect data from form elements • How to process the data you collect

Collecting Data from HTML Forms First let’s briefly look at how to use HTML form elements before going on to look at how JavaScript manipulates the objects associated with those elements.

Tip

Use HTML/XHTML tables to position form elements on a Web page. Web browsers ignore whitespace so positioning form elements tidily on the page can be a gamble, if you don’t use tables or Cascading Style Sheet positioning. In this chapter we will use tables.

Listing 6.1 creates a simple XHTML form that makes use of several form elements. We will discuss their use in a moment.

INPUT

LISTING 6.1 A Simple Form Using Several XHTML Form Elements (SimpleForm.htm)

HTML Forms and the String Object

LISTING 6.1

203

continued

XMML.com - Online Survey
Your Name:
Your Gender:		Male Female
Which of our consultancy services are you interested in?		XML  XSLT  SVG  XSL-FO  XForms
Which free gift would you prefer for filling out this survey?		<select name=”FreeGift”> Fresh Air A long life Contentment
Enter your comments in the text box		</td> </tr> <tr> <td> </td> <td> </td> <td><br /><input type=”submit” value=”Send Form”/></td> </tr> </table> </form> </body> </html><br /> <br /> Figure 6.1 shows the appearance on screen when Listing 6.1 is displayed in the Netscape 6 browser.<br /> <br /> 6<br /> <br /> 204<br /> <br /> Day 6<br /> <br /> OUTPUT FIGURE 6.1 An online survey using an XHTML form, shown in the Netscape 6 browser.<br /> <br /> Most of the elements contained in Listing 6.1 use the <input/> element. To create a text field we use the <input/> element with the type attribute that has a value of “text”. To create a radio button we use an <input/> element with the type attribute that has a value of “radio”. In the form we have two radio buttons, only one of which may be selected at a time. Each of the two radio buttons has the same value for the name attribute, which is possible because only one of the two is checked. Each radio button has a unique value attribute.<br /> <br /> ANALYSIS<br /> <br /> To create check boxes, we again use <input/> elements that, this time, have a type attribute with value of “checkbox”. In the form we have five checkbox <input/> elements each of which has a different name attribute.<br /> <br /> HTML Forms and the String Object<br /> <br /> 205<br /> <br /> The drop-down menu is created using a <select> element within which are nested a number of <option> elements. Each <option> element has a value attribute and it is the value of that value attribute which is sent to a server for processing. The text that is contained between the start and end tags of the <option> element is purely for human information on screen. Finally, in the data entry part of our form we have a text area, defined by a <textarea> element. The rows attribute defines the text area as having 5 rows and the cols attribute defines it as having 50 columns. The final part of the form is yet another <input/> element. This time it has a type attribute with the value of “submit”; therefore it is the submit button. So, if we can achieve all this form-related functionality using XHTML alone, the question comes to mind: Why we would want to use JavaScript in forms anyway?<br /> <br /> Why Use JavaScript in Forms? The form created in Listing 6.1 can, if used with HTML alone, create several problems. What happens, for example, if a user leaves blank the name text field? It makes it impossible to sensibly store the data according to user name. The form has no default selection for the radio buttons, so a user could submit the form without having any admitted gender. So we need, somewhere, to ensure that these two vital pieces of data have been entered. The check boxes (about consultancy services) don’t need any boxes to be checked, but we might want to remind users that services are offered. Checking that the text field has been filled in and that a radio button for gender has been selected could be done on the server-side. A disadvantage of that is that there could be a significant delay, depending on the speed of the user’s Internet connection, before an error message is delivered to the user. It makes a lot of sense to check for appropriate filling out of a form on the client side. Let’s move on to the syntax we need to know in order to access a form using JavaScript, and look at its contained elements.<br /> <br /> Accessing Form Elements Using JavaScript In Chapter 5, “An Introduction to Arrays,” you saw how the individual forms on a page could be accessed through the shortcut provided by the forms collection. So, if you had a form element with the name attribute set to “MyForm” and it was the second form on a Web page, then you would have three options for a syntax to access that form, as shown here:<br /> <br /> 6<br /> <br /> 206<br /> <br /> Day 6<br /> <br /> document.forms[1] document.forms[“MyForm”] document.forms.MyForm<br /> <br /> Collections provide a convenient shortcut when you need to access certain types of objects. Forms however are exceptional. When you want to access a form you don’t need to refer to the particular form through the forms collection—you can simply omit mentioning the collection! Forms and their elements have been scriptable since the first version of JavaScript was released with Netscape Navigator version 2. At that time, JavaScript was intended only to allow scripting of a limited number of elements, forms being important ones. Also at that time, allowing access to all elements and attributes through a hierarchically structured document object model (DOM) of an HTML document was not available for use. Forms were originally accessible simply by name from the document object. This technique of accessing a form by its name has persisted, in order to ensure backward compatibility; therefore, to access a form that has a given name, you simply would write: document.formName;<br /> <br /> Of course, for the above syntax to work correctly the form element must have a precisely corresponding name attribute, like so: <form name=”formName” ...><br /> <br /> Note that forms are the only elements of a Web page that can be accessed in this way. All other elements must be accessed through their collections; or, if not part of a collection, it must be accessed through one of the W3C DOM methods you will learn about in Chapter 8, “The Browser Issue.” In this chapter, we will examine how to access the objects that represent the elements in a form in order to obtain or manipulate the data they contain. Some form elements are relatively straightforward to access and some are more involved, requiring the use of arrays and special properties. But, as you will see, all are scriptable. Let’s look at using JavaScript with all the child objects of a form such as text fields, text areas, radio buttons, and select boxes. First, though, let’s look at the form element itself.<br /> <br /> Properties of the <form> Element The start tag of a <form> element can contain several attributes that identify the form and define its behavior. These attributes are reflected in some of the properties of the form object that JavaScript makes available to us.<br /> <br /> HTML Forms and the String Object<br /> <br /> 207<br /> <br /> The properties that belong to a form naturally include the form’s name. If it didn’t we wouldn’t be able to access it by name using the technique described above. In addition to the name property, we can also access the following properties of the form object: •<br /> <br /> acceptCharset<br /> <br /> •<br /> <br /> accessKey<br /> <br /> •<br /> <br /> action<br /> <br /> •<br /> <br /> encoding<br /> <br /> •<br /> <br /> enctype<br /> <br /> •<br /> <br /> length<br /> <br /> •<br /> <br /> method<br /> <br /> •<br /> <br /> tabIndex<br /> <br /> •<br /> <br /> target<br /> <br /> If you are familiar with the HTML <form> tag, you may recognize that many of these properties reflect attributes that can be added to the start tag of a <form> element. However take note of the case of the letters used in JavaScript for the property names. In HTML it has been common practice to spell an attribute’s tag name using uppercase letters. Now with the arrival of XHTML, it is becoming more common practice to write attribute names using lowercase letters. In JavaScript, you do not have flexibility in how property names should be written. JavaScript property names are case sensitive so it is important to write them correctly within <script> tags. In general, if an attribute name is a single word, then its corresponding JavaScript property name is simply written fully in lowercase letters. But if a property or method name consists of more than one word, then it is written with the first letters of all but the first word capitalized. Therefore, for example, the HTML attribute tabindex is written as tabIndex in JavaScript. Listing 6.2 illustrates the syntax for accessing and displaying a number of the properties of the Form object for the MyName form shown in Listing 6.1.<br /> <br /> 6 INPUT<br /> <br /> LISTING 6.2<br /> <br /> Using document.write to Display the Values of Properties of a Form Object (SimpleForm02.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”> <html> <head> <title>XMML.com - Online Survey</title> </head><br /> <br /> 208<br /> <br /> Day 6<br /> <br /> LISTING 6.2<br /> <br /> continued<br /> <br /> <body> <form action=”http://www.XMML.com” method=”POST” name=”MyForm”> <table width=”600”> <tr><th colspan=”3” align=”center”>XMML.com - Online Survey<br /><br /></th> </tr> <tr> <td>Your Name:</td> <td> </td> <td><input type=”text” name=”YourName”/></td></tr> <tr> <td>Your Gender:</td> <td> </td> <td> <input type=”radio” name=”Gender” value=”Male”/>Male<br /> <input type=”radio” name=”Gender” value=”Female”/>Female<br /> </td> </tr> <tr><td>Which of our consultancy <br />services are you interested in?</td> <td align=”right”>  </td> <td> <input type=”checkbox” name=”XML”/> XML<br /> <input type=”checkbox” name=”XSLT”/> XSLT<br /> <input type=”checkbox” name=”SVG”/> SVG<br /> <input type=”checkbox” name=”XSL-FO”/> XSL-FO<br /> <input type=”checkbox” name=”XForms”/> XForms<br /> </td> </tr> <tr> <td>Which free gift would you prefer for filling out this survey?</td> <td> </td> <td> <select name=”FreeGift”> <option value=”Choice1”>Fresh Air</option> <option value=”Choice2”>A long life</option> <option value=”Choice3”>Contentment</option> </select> </tr> <tr> <td>Enter your comments in<br />the text box </td> <td> </td> <td><textarea name=”Comments” rows=”5” cols=”50”/></td> </tr> <tr> <td> </td> <td> </td><br /> <br /> HTML Forms and the String Object<br /> <br /> LISTING 6.2<br /> <br /> 209<br /> <br /> continued<br /> <br /> <td><br /><input type=”submit” value=”Send Form”/></td> </tr> </table> </form> <script type=”text/javascript” language=”javascript”> <!-- // document.write(“<h3>The properties of the <i>” + document.MyForm.name + “</i> form:</h3>”); document.write(“<p>The action property has the value: <b>” + document.MyForm.action + “</b></p>”); document.write(“<p>The method property has the value: <b>” + document.MyForm.method + “</b></p>”); document.write(“<p>The name property has the value: <b>” + document.MyForm.name + “</b></p>”); document.write(“<p>The length property has the value: <b>” + document.MyForm.length + “</b></p>”); document.write(“<p>The enctype property has the value: <b>” + document.MyForm.enctype + “</b></p>”); // --> </script> </body> </html><br /> <br /> Figure 6.2 shows the new output that has been added to the document displayed in a browser window. In Internet Explorer the value of the enctype property is explicitly shown as “undefined”; whereas in Netscape 6 it is displayed as a blank. If you examine the code towards the end of Listing 6.2 you will see that we used to output values of, for example, the action property, which is the representation in the DOM of the action attribute. The value for the enctype property is shown to be undefined because we have not defined a value for it within the <form> element’s start tag.<br /> <br /> ANALYSIS<br /> <br /> document.write<br /> <br /> Let’s spend a moment clarifying why the length property is displayed as having a value of 11. The length property represents the number of form elements that are children of the <form> element. Thus we have one <input> element of type text, two <input> elements of type radio, five <input> elements of type checkbox, one <select> element, one <textarea> element, and one <input> element of type submit. In total, there are 11 elements. The <option> elements nested within the <select> element are not part of the length property since they are not children (although they are descendants) of the <form> element.<br /> <br /> 6<br /> <br /> 210<br /> <br /> Day 6<br /> <br /> OUTPUT FIGURE 6.2 Values of the form property displayed in the Netscape 6 browser.<br /> <br /> Many form attributes can be changed simply by assigning a new value to the corresponding property names; you can change the action attribute of a form based on a choice a user makes. In this way the data entered into a form could be submitted to a different URI for different processes to be carried out. Assuming that this change has been detected (we will see how later), we could change the action property of a form using the following statement: document.formName.action = “http://www.xmml.com/new/page.htm”;<br /> <br /> The same goes for all of the other form attributes that have a corresponding property that belongs to the Form object. The exception is the length property, which is read-only— because it is determined by the number of child elements of the Form object, as we have just discussed.<br /> <br /> HTML Forms and the String Object<br /> <br /> 211<br /> <br /> Methods of the Form Object Forms have two JavaScript methods that can be applied to them to reflect the standard “submit” and “reset” buttons that can be added to a form using HTML/XHTML input tags. In fact both these methods have the same names as the buttons: submit() and reset(). Note again the case of the letters that make up the names of these methods— JavaScript won’t recognize them if you don’t spell them using lowercase letters. Both the reset() and submit() methods are applied to an HTML form by navigating programmatically to the form’s object, and then applying them with the dot notation. For example, to submit a form you would write: document.formName.submit();<br /> <br /> Or, to reset a form using JavaScript’s reset() method, you could write: document.formName.reset();<br /> <br /> If you intend to use the submit() method to submit a form on one of your Web pages, then make sure that it is clear to the user that the form will submit. If the user wasn’t expecting the form to be submitted, then it is possible that he will hit the stop button on his browser, which could cause problems on the server side with half-submitted information. The intended use of the submit() method was to allow designers to replace the standard button (which some consider to be ugly), with an image that fits nicely into a site’s design. The reset button should also be used with care. If a visitor to your site has filled out all the information in a (possibly lengthy) form, and it is suddenly wiped because you have made some sort of obscure use of the reset() method, they may not try again. The most obvious use of the reset() method is to create your own reset buttons using images rather than the standard buttons. Alternatively you can change the appearance of the standard buttons using cascading style sheets. When changing properties in a form you should be aware of a potential problem that could arise if the user chooses to refresh the page. Most JavaScript induced changes are erased during a page refresh; whereas, with the soft refresh caused by simply clicking the browser’s refresh button, data entered into the form is not erased. If some information the user had entered were to change the action property of a form and the user then refreshed the page, the action would be changed back to its original value; however, the choice made in the form that should cause the script to change the action property would still exist, but without the action property having been appropriately changed. The form could then end up being submitted to the wrong URI for processing, causing unpredictable and likely undesirable results.<br /> <br /> 6<br /> <br /> 212<br /> <br /> Day 6<br /> <br /> One of the ways around this potential problem would be to create a script that would check for any entries in a form each time the page loaded. It could then act accordingly and make any necessary changes to the form’s properties. However, the process of detecting user changes to a form that have persisted through a refresh can be involved and can be as simple or complex as the forms themselves. A much easier way is simply to reset the form entirely. To make sure our forms are blank every time the page loads, you will need to use an event handler. Resetting the forms on a page when the page loads is a fairly simple task, as shown in Listing 6.3.<br /> <br /> INPUT<br /> <br /> LISTING 6.3<br /> <br /> Resetting a Form When a Web Page Loads (ResetForm.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”> <html> <head> <title>Resetting a form on page load</title> <script type=”text/javascript” language=”javascript”> <!-- // function ClearForm(){ document.MyForm.reset(); } // --> </script> </head> <body onload=”ClearForm()”> <table> <form name=”MyForm” action=”http://www.XMML.com/” method=”Post”> <tr> <td width=”15%” align=”right”>First Name: </td> <td> <input type=”text” name=”FirstName”/></td> </tr> <tr> <td align=”right”>Last Name: </td> <td> <input type=”text” name=”LastName”/></td> </tr> <tr> <td align=””right”>User Number: </td> <td> <input type=”text” name=”UserNo”/></td> </tr> <tr> <td> </td> <td> <input type=”submit” value=”Submit Form”/></td><br /> <br /> HTML Forms and the String Object<br /> <br /> LISTING 6.3<br /> <br /> 213<br /> <br /> continued<br /> <br /> </tr> </table> </form> </body> </html><br /> <br /> On the <body> element notice that there is an onload attribute which has the value of “ClearForm()”. When the page loads, the onload event handler is called. The value of the onload attribute specifies that the ClearForm() function is to be called. The only action of the ClearForm() function is to execute the reset() method on the MyForm form.<br /> <br /> ANALYSIS<br /> <br /> You may remember from Chapter 4, “JavaScript Is Object-Based,” that if you changed the Object Explorer page to explore the properties and child objects of an object other than the navigator object, you would likely see names beginning with the two letters “on”. We mentioned at the time that the name for these was “event handlers.” The reason for this is that they “handle,” or intercept, events that occur due to a user’s actions. If there is more than one form in a Web page, we could modify the ClearForm()function to reset all the forms on the page, as shown here: function ClearForm() { for (var i=0; i<document.forms.length; i++) { document.forms[i].reset(); } }<br /> <br /> An alternate syntax is to omit the onload attribute from the start tag of the <body> element and use onload directly within the <script> element: onload = function() { for (var i=0; i<document.forms.length; i++) { document.forms[i].reset(); } }<br /> <br /> The loading of a Web page isn’t the only event that JavaScript can process. If we use an event handler to capture a click on a button, then we can process that event using a function of our own design. Listing 6.4 shows a very simple form that applies the form object’s reset() method when a button is clicked and then outputs an alert box that tells the user that the reset has been carried out.<br /> <br /> 6<br /> <br /> 214<br /> <br /> Day 6<br /> <br /> INPUT<br /> <br /> LISTING 6.4<br /> <br /> Using the Form Object’s reset() Method to Reset a Form in Response to User Input (TinyForm.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”> <html> <head> <title>Test for the reset() method</title> <script type=”text/javascript” language=”javascript”> <!-- // function MyReset(){ document.TestForm.reset(); alert(“Your form has been reset!”); } // --> </script> </head> <body> <form name=”TestForm”> <table> <tr> <td width=”15%” align=”right”>Your Name:</td> <td ><input type=”text” name=”Name”/></td> </tr> <tr> <td align=”right”>Your Gender:</td> <td><input type=”text” name=”Gender”/></td> </tr> <tr> <td width=”15%”><input type=”button” value=”Submit”/></td> <td><input type=”button” value=”Reset” onclick=”MyReset()”/></td> </tr> </form> </body> </html><br /> <br /> Notice in the second <input> element that there is an onclick attribute. The value of the onclick attribute is “MyReset()”, in other words the MyReset() function is called. The MyReset() function makes use of the reset() method of the Form object named “TestForm”:<br /> <br /> ANALYSIS<br /> <br /> document.TestForm.reset();<br /> <br /> Note<br /> <br /> Some sources of information state that Netscape 6 does not execute the reset() method. If you run Listing 6.4 in a Netscape 6.0 or later browser, you will see that the form is reset correctly.<br /> <br /> HTML Forms and the String Object<br /> <br /> 215<br /> <br /> Event handlers will be discussed in more detail in Chapter 10, “Events and Events Handling.” The code in Listing 6.4 allows you to create your own submit button. You could have some text that calls the submit() method of a form, as in the listing, but more likely you will want to use an image to achieve a more attractive appearance than the standard gray button.<br /> <br /> Tip<br /> <br /> If you want to disable a submit or reset button, then you can do so using a function that returns the boolean value false.<br /> <br /> Let’s move on to look in a little more detail at the HTML/XHTML elements within a form—we will call them “form elements”—and how we can manipulate them using JavaScript.<br /> <br /> Form Elements Forms can have, in principle, any number of child elements of several different types. Form elements can be split into three different groups: • Text-based elements • Selection-based elements • Control elements Text-based elements include textboxes, text areas, hidden inputs, and the file-upload element. Selection-based elements include check boxes, radio buttons, and select boxes. Control elements include buttons, for example. Note that only certain properties of form elements can be changed. Some are read-only. As a general rule, if the page needs to be restructured in order to implement a change made by a script, then that change is not allowed. For example, the type attribute of the <input> element is read-only. If you were able to change an <input> element’s type attribute from “button” to “radio” for example, the page would need to be restructured. Similarly the defaultValue property is also read-only. On the other hand, the name property of any form element is read/write. The page does not need to be redrawn to implement a change that might be made to an element’s name, so you can both read and write an element’s name property.<br /> <br /> 6<br /> <br /> 216<br /> <br /> Day 6<br /> <br /> Caution<br /> <br /> Be careful if you change the name property of a form element. If any of your other code makes use of the original value of the name property before you made a change, then the output of some parts of your code could be unpredictable.<br /> <br /> Textual elements do, however, allow their content, and hence appearance, to be changed. This includes not only changes made by a user but also changes made by a script. Therefore we can change the value of a text area to change its content, or even change the value of a button to change the text it shows.<br /> <br /> Caution<br /> <br /> Netscape Navigator 4 does not resize buttons if you alter the value of the value property of a button object with the intention of relabeling the button. Any text that is longer than the original length that you assign to the value property will simply be truncated if it is too large for the existing button dimensions.<br /> <br /> The elements Collection As well as accessing each element of a form by name, it is possible to access all of them through a collection of the document object. Each form that belongs to the forms collection contains another collection called the elements collection. This collection brings all the elements in the form together so as to make them accessible as an array. As with all the other collections, it doesn’t matter how these elements are nested within other HTML elements. If for example a table has been used inside the form to lay out certain elements, the elements collection will skip through the table straight to the elements you need to access. The elements collection orders the elements of a form in the order in which they were written into the XHTML/HTML that makes up the page. Therefore, you could in theory access the third element of the second form on a page by using this notation, as shown here: document.forms[1].elements[2]<br /> <br /> However, as with the forms collection, it is recommended that you not use the elements collection as your main means of accessing the elements of your forms. It can be fragile. Suppose you insert an additional <input> element early in a form, then your code for all later form elements will no longer work correctly because they now are in a different position in the elements array.<br /> <br /> HTML Forms and the String Object<br /> <br /> 217<br /> <br /> If you choose to use several forms on the same Web page, you can access the first form and last form. To access the first form on the page use the following: document.forms[0]<br /> <br /> To access the last form on the page use this syntax: document.forms[document.forms.length-1]<br /> <br /> To correctly use the syntax for both those techniques you must remember that the first form is numbered from zero. So far we have accessed forms through the document object and referred to the form by name. But when a script or function is called from the form or an element in the form itself we can use a shortcut. This shortcut is made available through a special JavaScript keyword that refers to the object that the script was called from: the keyword this. When the keyword this is used in a script contained within a tag, it is that tag’s object that it points to. This can save you from navigating through the hierarchy of objects as well as shorten the amount of code you have to write. Whether you decide to pass a reference to the present object (using this), its form (this.form), or one of its properties (this.property) depends on what you are trying to achieve. In a script block the this keyword sometimes isn’t really the appropriate approach. If you can avoid referring to the window object, then you can also avoid using the keyword this. When used in other locations, the object it refers to is different and so it can be very useful. Listing 6.5 shows a simple example that uses the this object in the value of the onfocus attribute of the <input> element.<br /> <br /> INPUT<br /> <br /> LISTING 6.5<br /> <br /> Using the this Object within an onfocus Attribute (This.htm)<br /> <br /> <html> <head> <title>Testing this</title> </head> <body> <h1>Testing this</h1> <form action=”collect.php”> <input type=”text” name=”emailAD” size=”15” value=”Email Address” onfocus=”this.value=’’”> <br /><submit value=”Subscribe”><br /> <br /> 6<br /> <br /> 218<br /> <br /> Day 6<br /> <br /> LISTING 6.5<br /> <br /> continued<br /> <br /> </form> </body> </html><br /> <br /> In the example in Listing 6.5, there is a text box where the information to be entered is indicated by the text it contains: “Email Address”. When space is limited this technique can save you from using extra text outside the element explaining what it is for and taking up space. Clearly anyone who wants to send his e-mail address to subscribe has to delete this text from the text box first—unless we do it for the user. And that is exactly what the onFocus event handler that has been placed in the <input> tag does. It removes the text “Email Address” by assigning an empty string to the value of the text box. The neat thing about this is that you can use the keyword this instead of a longer reference:<br /> <br /> ANALYSIS<br /> <br /> document.forms[0].emailAD.value<br /> <br /> And if you wanted to access the form using its name, as recommended, then you would also have to give the form and text box a name. The this keyword bypasses all this however. By using it you are saved from lengthier references that include the document and form name in the object path, as well as sometimes being saved from naming the form concerned. When calling functions using event handlers located in the opening tags of an element this can be very useful. The this keyword can be used to send data more than once. For example you may wish to send an element’s name and its value to a function. To do that we would simply write: onclick=”myFunc(this.name, this.value)”<br /> <br /> You can also send references to other elements in a form by using the keyword this in conjunction with the form property (which references the parent Form object) provided by each element to navigate to another element within the form: this.form.otherElement.value<br /> <br /> The above syntax refers to the parent Form object of the this object and then finds another form element named “otherElement” and finally finds the value of that other form element.<br /> <br /> HTML Forms and the String Object<br /> <br /> 219<br /> <br /> Properties Common to Several Form Elements All form elements have a form property, which contains the identity of the Form object within which the form element is contained. You have just seen how it can be used. Form elements also typically have a name property, which corresponds to the name attribute on the start tag in the HTML. When we come to validate data entered into a form, we must be able to access the value entered. We will look in a moment at how that is done, as well as some other issues relevant to individual types of the form element. The property belonging to text-based form elements that we will make most use of is the value property. Using JavaScript we can get and set the text contained in text-based elements using this property, with the exception of one: elements with the type “file”. File upload elements, as we will refer to them from now on, cannot be set using scripts. If we could then we would be able to upload files from the computer of anyone visiting our site simply by filling in these elements with a likely path to a file. In fact, the element could be hidden so that the user wouldn’t even be aware that the file-upload element was on the page. Obviously this could enable serious security breaches and invasions of privacy, hence the restriction. To access a form element by name, the element must, of course, have a name attribute in its start tag. Even if the element is nested inside other elements such as a table, we can still access it as if it was a direct child element of the form. Again this is because forms have been around for a long time and shortcut routes to an object were the original means of accessing elements. Hence we can access a text box by simply writing something similar to the following: document.formName.textboxName<br /> <br /> However, this is not enough to access the text in the text box. The text box is still a full object, and the text it contains is one of its properties. You may already have guessed that if you can place text in a text box by giving it a value attribute in its opening tag that this will be the property name used to access it as well. If you did then you are absolutely correct. To access a text box and assign the text it contains to a variable, you would write a statement like the following one: var myText = document.formName.textboxName.value;<br /> <br /> The text that is in the text box we have addressed will now also have been assigned to the variable myText. Listing 6.6 shows a simple example.<br /> <br /> 6<br /> <br /> 220<br /> <br /> Day 6<br /> <br /> INPUT<br /> <br /> LISTING 6.6<br /> <br /> Retrieving Text Using the value Property (ValueProperty.htm)<br /> <br /> <html> <head> <title>Retrieving Text</title> <script language=”javascript” type=”text/javascript”> <!-function alertText() { var text = document.simpleForm.myText.value; alert(text); } //--> </script> </head> <body> <h1>Retrieving Text</h1> <form name=”simpleForm”> <input type=”text” name=”myText” size=”20” /> <input type=”button” value=”Alert Text” onclick=”alertText()” /> </form> </body> </html><br /> <br /> The onclick attribute that has been added to the start tag of the <input> element is of type button. The onclick attribute is an event handler; however, whereas the onload event handler triggers when the page loads, the onclick event handler detects when a user clicks on the button. In the listing it calls the function alertText() when users click on the button. Note the syntax of the onclick event handler, as we will use it often when using buttons to call a function.<br /> <br /> ANALYSIS<br /> <br /> Inside XHTML tags we write event handlers completely in lowercase for compliance with the XHTML recommendation. In HTML they are often written with uppercase only. This is not an option for event handler names in JavaScript. Again, JavaScript is case sensitive, and if we assign an event handler using JavaScript we must use correct case letters.<br /> <br /> HTML Forms and the String Object<br /> <br /> 221<br /> <br /> Once the button on the page has been clicked and the function alertText() is called, the first thing that happens is that the function retrieves the text from the text box. It does this through the first statement. In this statement we navigate using the document object to the form called simpleForm, and then to the text box in this form called myText and finally to myText’s value property. The value of the value property when found is assigned to the variable called text before the next line when the alert() function is used to bring up an alert box that contains its value for us to see.<br /> <br /> Caution<br /> <br /> One important point to note about values retrieved from forms is that they have the data type string—even if the characters in the form element concerned were purely numeric. Care must be taken to ensure that the necessary conversions are made if the values are to be used in a process where they won’t be converted automatically. We will come back to this point as it arises.<br /> <br /> The text Element This corresponds to an <input> element with the type attribute having a value of “text”. Getting access to the value entered into a text element is straightforward since the text element is a child of the form and the text element has a value property.<br /> <br /> INPUT LISTING 6.7<br /> <br /> Listing 6.7 shows how you can access the value entered into a text field.<br /> <br /> Getting the Value Entered into a Text Field (TextValue.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Displaying the value entered in a text field</title> <script type=”text/javascript” language=”javascript”> <!-- // function DisplayValue(){ if(document.MyForm.MyTextField.value!==””){ alert(“The value entered was \n” + document.MyForm.MyTextField.value); } else{ alert(“The text field was empty!\nPlease enter your name.”); } }<br /> <br /> 6<br /> <br /> 222<br /> <br /> Day 6<br /> <br /> LISTING 6.7<br /> <br /> continued<br /> <br /> // --> </script> </head> <body> <form name=”MyForm” action=”http://www.XMML.com/” method=”Post” onsubmit=”DisplayValue()”> <input type=”text” name=”MyTextField”/><p>Enter your name</p> <input type=”submit” value=”Click to Submit”/> </form> </body> </html><br /> <br /> Notice on the <form> element the onsubmit attribute which calls the DisplayValue() function. The DisplayValue() function uses the name of the form and the name of the text field in the value which is alerted. However, if the value in the text field was the empty string, then a message is displayed, indicating to the user that they need to enter their name into the text field.<br /> <br /> ANALYSIS<br /> <br /> This code only takes us part way to what we want—we can check the value of the value property but that does not prevent the form being submitted. If you run the code then you will find that even though the data in the text field is invalid the form is still submitted. We will, later in the chapter, look at how we can prevent that from happening. First, let’s improve on another minor deficiency of the form we just looked at. When the form loads, try typing your name. What happens? Nothing, because the focus is not on the text field. So when you type, the text isn’t entered into the field. Listing 6.8 uses the onload event to attach focus to the text field, so you can start typing without first moving your mouse to the text field and clicking there to give it focus.<br /> <br /> INPUT<br /> <br /> LISTING 6.8 Using JavaScript to Attach Focus to a Selected Text Field to Help Usability (TextFieldWithFocus.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Displaying the value entered in a text field</title> <script type=”text/javascript” language=”javascript”> <!-- // onload = function(){ document.MyForm.MyTextField.focus(); }<br /> <br /> HTML Forms and the String Object<br /> <br /> LISTING 6.8<br /> <br /> 223<br /> <br /> continued<br /> <br /> function DisplayValue(){ if(document.MyForm.MyTextField.value!==””){ alert(“The value entered was \n” + document.MyForm.MyTextField.value); } else{ alert(“The text field was empty!\nPlease enter your name.”); } } // --> </script> </head> <body> <form name=”MyForm” action=”http://www.XMML.com/” method=”Post” onsubmit=”DisplayValue()”> <input type=”text” name=”MyTextField”/><p>Enter your name</p> <input type=”submit” value=”Click to Submit”/> </form> </body> </html><br /> <br /> You can see within the <script> element the function which is associated with the onload event. You can use the focus() method of the text property of the form object to attach focus to the desired field. Doing that is more convenient to the user.<br /> <br /> ANALYSIS<br /> <br /> The textarea Element Text areas are represented in HTML by the <textarea> element, and are intended for the entry of free text. The dimensions of the text area are specified by the values of the rows and cols attributes. Text areas could be used, for example, to collect customer feedback comments or for detailing delivery instructions for an online purchase. If you want to access the value of a text area, you can use syntax similar to that which was used for the text field shown earlier. For example, if you have a text area with a name attribute of value “MyTextArea” then you can access the value of the text area as follows: document.MyForm.MyTextArea.value<br /> <br /> In addition to simply gaining access to the value of a text area, you can control whether or not it is accessible to users, depending on what they have done in other parts of the form. In some circumstances, you may want to disable some form elements to ensure that data has been entered elsewhere in the form before allowing the user to enter comments, for example.<br /> <br /> 6<br /> <br /> 224<br /> <br /> Day 6<br /> <br /> Listing 6.9 shows an example where users must enter their names into the form before the text area is enabled for them to enter their comments. We will set the text area to read-only until the text field for the user’s name has had some text entered into it.<br /> <br /> INPUT<br /> <br /> LISTING 6.9 Restricting a User to the Text Field for Their Name until They Enter Some Text (TextAreaReadonly.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Disabling a text area until another field has been completed</title> <script type=”text/javascript” language=”javascript”> <!-- // onload = function(){ document.MyForm.MyTextField.value=””; // document.MyForm.MyTextField.focus(); // sets focus to MyTextField document.MyForm.Comments.readOnly = true; // sets text area to read-only } // end onload function DisplayValue(){ if(document.MyForm.MyTextField.value!==””){ alert(“The value entered was \n” + document.MyForm.MyTextField.value); } else{ alert(“The text field was empty!\nPlease enter your name.”); } }// end function DisplayValue() function ChangeStatus(){ if (document.MyForm.MyTextField.value!==””){ document.MyForm.Comments.readOnly = false; document.MyForm.Comments.select(); } // end if else{ document.MyForm.MyTextField.focus(); } //end else } function CheckStatus(){ if (document.MyForm.MyTextField.value==””){ alert(“First please enter your name”); document.MyForm.MyTextField.focus(); } // end if else{ document.MyForm.Comments.select(); } // end else } // end function CheckStatus()<br /> <br /> HTML Forms and the String Object<br /> <br /> LISTING 6.9<br /> <br /> 225<br /> <br /> continued<br /> <br /> // --> </script> </head> <body> <table> <form name=”MyForm” action=”http://www.XMML.com/” method=”Post” onsubmit=”DisplayValue()”> <tr> <td><input type=”text” name=”MyTextField” onblur=”ChangeStatus()”/></td> <td>Enter your name</td> </tr> <tr> <td><textarea name=”Comments” rows=”6” cols=”40” onmouseup=”CheckStatus()” >Please enter your name first	Enter your comments.

In Listing 6.9, we are really ensuring that the user filled in his name before he did anything else in the form. When the document loads we set focus to the element named “MyTextField”. If the user tries to tab out of there or clicks on another part of the Web page, then the text field will lose focus—that is, “blur.” The element has an onblur attribute which sets the focus back to the text field if no text has been entered; or, if text has been entered, sets the text area to no longer readonly, and puts the focus on the text area. If the user tries to get into the text area (which is read-only) by clicking on it before the user’s name has been entered, the onmouseup attribute of the element calls the CheckStatus() function and sets focus back to the MyTextField text field if no name has been entered.<br /> <br /> ANALYSIS<br /> <br /> You may not want to tie down users this tightly, but it can be useful to think about how to control what the user can do using JavaScript. Historically, text areas used to have text wrap turned off by default. If that was what you wanted then great, but if you didn’t then it could be problematic, depending on whether you were scripting for older browsers or not. The reason for this is that different browsers recognized different keywords for turning on text wrap. Because it is not possible to assign an attribute two different values for two different browsers using only<br /> <br /> 6<br /> <br /> 226<br /> <br /> Day 6<br /> <br /> HTML, JavaScript became a workaround for detecting which browser was being used. Browser detection is discussed in Chapter 8. Let’s move on to looking at the elements where the user has to make a choice.<br /> <br /> Check Boxes The check box is the easiest choice-based form element to deal with and use. The two main pieces of information you may want to know about a check box are its value and whether it is checked or not. The value assigned to the value attribute of a check box can be obtained or set through the value property of the Checkbox object. The technique is similar to the one we have looked at for text fields and text areas: var valueChecked = document.MyForm.someCheckbox.value<br /> <br /> Finding out whether a check box is checked or not is also straightforward. Just as the name used for the attribute that causes a check box to start off checked when the page loads is the name “checked”, the corresponding property of a Checkbox object is also called checked. The checked property of a Checkbox object can have one of two values. A checkbox is either checked or it isn’t, so the checked property can have the value true or false. For example: var chkStatus = document.formName.checkboxName.checked;<br /> <br /> assigns the value true to the variable chkStatus if the check box being examined is checked, but false if it isn’t. If we have several check boxes in a form it becomes a little cumbersome to write code to check each of them by name. An alternative approach is to loop through the elements array of the form object and check whether or not each element is an <input> element of type checkbox. Listing 6.10 shows this being done in the context of the online survey we saw earlier. Run the code with none, some, or all of the check boxes checked to see the results.<br /> <br /> INPUT<br /> <br /> LISTING 6.10<br /> <br /> Checking and Displaying Whether a Form Element Is a Check Box and Displaying Whether It Is Checked or Not (SimpleFormCheckboxes.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”> <html> <head> <title>XMML.com - Online Survey</title> <script type=”text/javascript” language=”javascript”> <!-- // function CheckCheckboxes(){<br /> <br /> HTML Forms and the String Object<br /> <br /> LISTING 6.10<br /> <br /> 227<br /> <br /> continued<br /> <br /> var elLength = document.MyForm.elements.length; for (i=0; i<elLength; i++) { var type = MyForm.elements[i].type; if (type==”checkbox” && MyForm.elements[i].checked){ alert(“Form element in position “ + i + “ is of type checkbox and is checked.”); } // end if else if (type==”checkbox”) {alert(“Form element in position “ + i + “ is of type checkbox and is not checked.”); } // end else if else {} // do nothing if not of type checkbox } // end for } // end function CheckCheckboxes() // --> </script> </head> <body> <form action=”http://www.XMML.com” method=”POST” name=”MyForm”> <table width=”600”> <tr><th colspan=”3” align=”center”>XMML.com - Online Survey<br /><br /></th> </tr> <tr> <td>Your Name:</td> <td> </td> <td><input type=”text” name=”YourName”/></td></tr> <tr> <td>Your Gender:</td> <td> </td> <td> <input type=”radio” name=”Gender” value=”Male”/>Male<br /> <input type=”radio” name=”Gender” value=”Female”/>Female<br /> </td> </tr> <tr><td>Which of our consultancy <br />services are you interested in?</td> <td align=”right”>  </td> <td> <input type=”checkbox” name=”XML”/> XML<br /> <input type=”checkbox” name=”XSLT”/> XSLT<br /> <input type=”checkbox” name=”SVG”/> SVG<br /> <input type=”checkbox” name=”XSL-FO”/> XSL-FO<br /> <input type=”checkbox” name=”XForms”/> XForms<br /> </td> </tr> <tr> <td>Which free gift would you prefer for filling out this survey?</td> <td> </td> <td><br /> <br /> 6<br /> <br /> 228<br /> <br /> Day 6<br /> <br /> LISTING 6.10<br /> <br /> continued<br /> <br /> <select name=”FreeGift”> <option value=”Choice1”>Fresh Air</option> <option value=”Choice2”>A long life</option> <option value=”Choice3”>Contentment</option> </select> </tr> <tr> <td>Enter your comments in<br />the text box </td> <td> </td> <td><textarea name=”Comments” rows=”5” cols=”50”/></td> </tr> <tr> <td> </td> <td> </td> <td><br /><input type=”submit” value=”Send Form” onclick=”CheckCheckboxes()”/></td> </tr> </table> </form> </body> </html><br /> <br /> When the “Send Form” button is clicked the CheckCheckboxes() function is called. Within that function we set up a for loop to examine each of the element types in the form’s elements array. We check whether or not the element in the form’s elements array is or is not of type checkbox. For each element that is a checkbox we display an alert which states the position of the checkbox in the elements array, and whether it is checked or not. If the member of the elements array is not a check box then we do nothing, as indicated by the else clause towards the end of the CheckCheckboxes() function.<br /> <br /> ANALYSIS<br /> <br /> Radio Buttons Radio buttons are slightly more complicated to deal with than check boxes. However, with the knowledge you gained from Chapter 5, “An Introduction to Arrays,” you already have the knowledge necessary to work with them.<br /> <br /> Tip<br /> <br /> If you want to ensure that a user has consciously checked one of a set of radio buttons, then leave them all unchecked when the page loads and then verify, before the form is submitted to the server, that a radio button has been checked. If no radio button has been checked, then provide a reminder for the user that one of the radio buttons provided must be checked.<br /> <br /> HTML Forms and the String Object<br /> <br /> 229<br /> <br /> Radio buttons are, as you perhaps know, grouped together by giving individual radio buttons the same name. Initially this may seem to create a problem if you want access to these elements using JavaScript. After all, if you have several elements in a form with the same name, then you can’t access any one of them simply by writing the following: document.formName.radioName<br /> <br /> If you were to do this how would the JavaScript interpreter know which of the elements with the same name you were referring to? The solution is that JavaScript treats radio button groups as an array. The above line of code would effectively address the array, and to access any of the individual elements you would need to specify an element index. The element that comes first in the HTML that makes up the page is the first element in the array, the element that comes second on the page is second in the array and so on.<br /> <br /> Caution<br /> <br /> Remember that the first element in an array is numbered as zero.<br /> <br /> The array representing a radio button group has all the properties that you would expect to find in the arrays that you create yourself. For example there is a length property for the array that tells you how many entries there are in the array, or in radio button terms, how many radio buttons there are in a radio button group. Each radio button in a group is an object in its own right with its own individual properties. There is no one property that you can examine to discover which radio button in a group of radio buttons is the radio button that has been selected by the user (if any). You will need to examine each button in turn to determine whether it is selected or not. Here the length property comes in useful once again. In Listing 6.11 we use the length property to allow us to loop through the array of radio button objects to determine which of them has been selected.<br /> <br /> INPUT<br /> <br /> LISTING 6.11<br /> <br /> Detecting Which Radio Button Is Selected (RadioCheck.htm)<br /> <br /> <html> <head> <title>Checking a Radio Button Group</title> <script language=”javascript” type=”text/javascript”> <!--// function evalGroup() {<br /> <br /> 6<br /> <br /> 230<br /> <br /> Day 6<br /> <br /> LISTING 6.11<br /> <br /> continued<br /> <br /> var group = document.radioForm.myRadio; for (var i=0; i<group.length; i++) { if (group[i].checked) break; } if (i==group.length) return alert(“No radio button is checked”); alert(“Radio Button “ + (i+1) + “ is checked.”); } //--> </script> </head> <body> <h1>Checking a Radio Button Group</h1> <form name=”radioForm”> Radio Button 1: <input type=”radio” name=”myRadio” /><br /> Radio Button 2: <input type=”radio” name=”myRadio” /><br /> Radio Button 3: <input type=”radio” name=”myRadio” /><br /> Radio Button 4: <input type=”radio” name=”myRadio” /><br /><br /> <input type=”button” value=”Eval Group” onclick=”evalGroup()” /> </form> </body> </html><br /> <br /> In this example we have a form consisting of four radio buttons and a button which, when clicked, calls a function. The button has the onclick event handler added to call the function evalGroup() when clicked. In the first statement of evalGroup(), we used a variable called group to store a reference to our radio button group. We created a for loop that loops through the radio group, but with a difference.<br /> <br /> ANALYSIS<br /> <br /> Whereas our loops previously have looped through every element in an array, in this loop we have set it up so that it loops from the start but it will stop looping as soon as it finds a checked element. We can safely do that since only one button of a radio button group can be checked. If we find a radio button that has been checked, then we know that we already have found the only radio button in the radio button group which is checked— and it is pointless to go on and examine the status of the other buttons in the group.<br /> <br /> Select Boxes Select boxes are the trickiest of the form elements to script. A select box contains child elements in the form of <option> elements, which isn’t the case for any of the other<br /> <br /> HTML Forms and the String Object<br /> <br /> 231<br /> <br /> form elements. To add to this complexity, select boxes can take the form of either a dropdown menu where one option can be chosen, or a scroll box where multiple options can be selected. The single choice drop-down menu is the most popular application, so let’s concentrate on it. As is the case for radio button groups, select boxes require array-handling skills. In the case of select boxes however, it is not the select box that is the array. It is the group of options that it contains that are in effect an array. In addition to this, each option is also an object in its own right. The selectedIndex property tells you which position in the array is occupied by the option object that has been selected. Now that we know how to find out which option object is the selected option in a select box, we need to discover how to access that option in order to find out its properties, such as its value or the text that it shows in the select box. We do not usually give the options in a select box a name, so initially you may not seem to have any way to reference the array of options. However, JavaScript auto generates another collection for just this purpose—the options collection. The options collection is just like the other collections we have seen so far. Just as they are generated automatically whenever a document is loaded into the browser, the options collection is automatically generated whenever an options element is present in or written into a form. Whereas collections like the forms collection belong to the document object, options collections belong to the select box object that they were created for. Therefore to access the third option of a select box you would write the following: document.formName.selectBoxName.options[2]<br /> <br /> Remember arrays start at 0, so the third element has the index value 2. The key piece of information to know in order to work with select boxes is that a select object’s <option> elements do not have a selected property. This means that you cannot iterate through each option in a select box in turn checking to see if it is selected, as we did for radio button groups. However, happily, the solution is even easier. The select object has a property that tells us which option is selected. This is the selectedIndex property. Therefore to discover which option in a select box is selected you would simply write: document.formName.selectBoxName.selectedIndex<br /> <br /> Once you are able to access a select box and its options, the two most important properties that you may want to discover are its value property and the text shown by the option in the select box. The value property is just as you have seen before. If you want<br /> <br /> 6<br /> <br /> 232<br /> <br /> Day 6<br /> <br /> to discover the value that has been assigned to the second option of a select box, you would simply write: document.formName.selectBoxName.options[2].value<br /> <br /> As it happens, discovering the text that an option displays in its select box is just as easy. It is contained in a property called the text property. So to access the text for the second option of a select box you would write: document.formName.selectBoxName.options[2].text<br /> <br /> The following code shows a function that would allow you to output the value and text properties of a selected option object, like so: function { var var var var var<br /> <br /> getSelected()<br /> <br /> select = document.myForm.dropDown; index = select.selectedIndex; selectedTxt = select.options[index].text; selectedVal = select.options[index].value; msg = “The “ + (index+1) + “ option is selected. \n”; msg += “It is the option “ + selectedTxt; msg += “ and has the value “ + selectedVal + “.”; alert(msg);<br /> <br /> }<br /> <br /> Listing 6.12 shows an example of how to display the selected option of a select object in a form.<br /> <br /> INPUT<br /> <br /> LISTING 6.12 Capturing and Displaying the Value of the Selected <option> Element (SelectedOption.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”> <html> <head> <title>Find the selected option</title> <script type=”text/javascript” language=”javascript”> <!-- // function CheckOption(){ var Selected = document.TestForm.PrizeChosen.selectedIndex; var SelectedOption = document.TestForm.PrizeChosen.options[Selected].value; alert(“The prize you have chosen is a “ + SelectedOption); } // end function CheckOption() // --> </script> </head> <body> <form name=”TestForm” action=”SomePage.htm” onsubmit=”CheckOption()”><br /> <br /> HTML Forms and the String Object<br /> <br /> LISTING 6.12<br /> <br /> 233<br /> <br /> continued<br /> <br /> <table> <tr> <td width=”15%” align=”right”>Your Name:</td> <td ><input type=”text” name=”Name”/></td> </tr> <tr> <td align=”right”>Your Gender:</td> <td><input type=”text” name=”Gender”/></td> </tr> <tr> <td width=”15%”><select name=”PrizeChosen”> <option value=”Ferrari”>Red Ferrari</option> <option value=”Lear Jet”>Lear Jet</option> <option value=”Dream Home”>Home to $200,000</option> </select> </td> <td> </td> </tr> <tr> <td width=”15%”><input type=”submit” value=”Submit the form”/></td> <td> </td> </tr> </table> </form> </body> </html><br /> <br /> Buttons Buttons can be used to initiate actions in response, for example, to clicking a button. In Listing 6.4 we used an onclick attribute on a button to call a function. Before we go on to look at how you can ensure that only valid data is sent to the server, let’s take a step aside to look at some aspects of the String object.<br /> <br /> 6<br /> <br /> The String Object In Chapter 1, “Getting the Basics Right,” we learned about JavaScript’s five data types. At that time we found out that any data in a format that the JavaScript interpreter didn’t understand had to be expressed as a string. By storing it in strings, no errors will occur and JavaScript won’t attempt to interfere with its structure. It will simply store the characters one after another and leave the choice of processing up to us—and for that of course we will need tools. Fortunately, the String object provides many methods to process the content of strings.<br /> <br /> 234<br /> <br /> Day 6<br /> <br /> When we write a series of characters surrounded by opening and closing quote marks, we are making use of the String object’s literal notation. When the JavaScript interpreter comes across a string literal that contains some characters (or none at all in the case of the empty string) it gives these characters the data type string and stores them. So to store a string in the variable myString we can use a simple JavaScript variable declaration and assignment statement, like so: var myString = “Mary had a little lamb”;<br /> <br /> It is also possible to explicitly create new instances of the String object to store a string: var myString new String(“Mary had a little lamb”);<br /> <br /> In modern browsers these two statements are functionally identical. Even the assignment of a string literal to the variable myString allows you to use all the String object’s methods or to access its properties. Listing 6.13 demonstrates that for a String object created using the assignment of a string literal to a variable, the length property of the String object is accessible.<br /> <br /> INPUT<br /> <br /> LISTING 6.13<br /> <br /> Displaying the length Property of a String Object (FirstString.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>XHTML 1.0 Transitional Template</title> <script type=”text/javascript” language=”javascript”> <!-- // onload = function(){ var myString = “Mary had a little lamb”; document.write(“<h3>A first string</h3>”); document.write(myString + “ has a length of “ + myString.length + “ characters.”); } // --> </script> </head> <body> </body> </html><br /> <br /> If you run the code in Listing 6.13 you will see that the length of the string literal “Mary had a little lamb” is 22 characters.<br /> <br /> HTML Forms and the String Object<br /> <br /> 235<br /> <br /> You know from earlier chapters how to concatenate strings together using the + or += operators, but in addition to building up strings, you often will need to extract data from them as well. Listing 6.14 demonstrates both techniques. LISTING 6.14<br /> <br /> INPUT<br /> <br /> Concatenating Strings Using the + or += Operators (Concatenate.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Concatenating Strings</title> <script type=”text/javascript” language=”javascript”> <!-- // onload = function(){ var myString = “Mary had a little lamb”; var mySecondString = “Its fleece was white as snow”; document.write(“<h3>Concatenating using a separate variable</h3>”); concatString = myString + “<br />” + mySecondString; document.write(concatString); document.write(“<br /><br />”); document.write(“<h3>Using the <b>+=</b> operator</h3>”); myString += “<br />”; myString += mySecondString; document.write(myString); } // --> </script> </head> <body> </body> </html><br /> <br /> 6 If you run the code you will see that the visual output is identical when using each technique of concatenation.<br /> <br /> String Properties The String object has only one property that you are likely to make use of on a regular basis. This property is the length property. There are another two properties, which are the constructor and prototype properties that we have mentioned also exist for the Array object. In this instance, let’s concentrate on the length property.<br /> <br /> 236<br /> <br /> Day 6<br /> <br /> The String object’s length property is similar to the length property that belongs to the Array object. Just as the length property of the Array object tells you how many elements there are in an array, the length property of the String object tells you how many characters exist in a string. This includes spaces and non-alphanumeric characters, for example: var myString = “Ten chars!”; alert(myString.length); // alerts 10<br /> <br /> At first sight the length property may seen to be very limited in its usefulness. After all there aren’t going to be many occasions when you will want to tell a visitor to your site how many characters make up this or that string. However, the length property is very useful when it comes to string manipulation. Just as the length property of the Array object can be used to help our scripts reliably determine where to find the end of an array, the length property is used to determine where a string ends. For string manipulation this information is often very useful.<br /> <br /> String Methods The String object provides a very large number of methods. Some of these methods will help you when working with your strings—they are listed below. Examples of how to use some of these methods will follow. •<br /> <br /> charAt()<br /> <br /> •<br /> <br /> charCodeAt()<br /> <br /> •<br /> <br /> concat()<br /> <br /> •<br /> <br /> fromCharCode()<br /> <br /> •<br /> <br /> indexOf()<br /> <br /> •<br /> <br /> lastIndexOf()<br /> <br /> •<br /> <br /> localCompare()<br /> <br /> •<br /> <br /> match()<br /> <br /> •<br /> <br /> replace()<br /> <br /> •<br /> <br /> search()<br /> <br /> •<br /> <br /> slice()<br /> <br /> •<br /> <br /> split()<br /> <br /> •<br /> <br /> substr()<br /> <br /> •<br /> <br /> substring()<br /> <br /> •<br /> <br /> toLocaleLowerCase()<br /> <br /> HTML Forms and the String Object<br /> <br /> •<br /> <br /> toLocaleUpperCase()<br /> <br /> •<br /> <br /> toLowerCase()<br /> <br /> •<br /> <br /> toSource()<br /> <br /> •<br /> <br /> toString()<br /> <br /> •<br /> <br /> toUpperCase()<br /> <br /> •<br /> <br /> valueOf()<br /> <br /> 237<br /> <br /> The way in which you apply String object methods is very flexible. You probably expect that it is possible to apply the methods to data containers that contain a string value. For example, you can write: var strVal = “My string”.valueOf()<br /> <br /> to assign the value of a string to the variable strVal. In fact, your applications can be more complex than this. You can apply multiple methods to a string, all on the same line of code, for example: var strUC = “My string”.valueOf().toUpperCase();<br /> <br /> This code takes the value of the string literal, converts it to uppercase, and assigns the uppercase string to the strUC variable. As usual when multiple operators that are the same or have the same operator precedence are used in an expression, the operands of the member operator (.) are evaluated from left to right. In the example above, the first expression to be evaluated is the string “My string”. Once the JavaScript interpreter has parsed this, it then applies the next operand to the right which in this case is the valueOf() method. This method, as we know in the case of literals, simply returns the string itself. Once this result from applying the valueOf() method has been calculated, then the next and final expression is applied to the result using the member operator. As its name implies, this string method converts to uppercase all the characters of the string to which it is applied. Hence the final value that is assigned to the variable strUC is the string “MY STRING”. Let’s move on now and look at how to use some of the String object’s methods.<br /> <br /> Simple Case Transformations with toUpperCase() When you need to compare strings you can run into the problem that lowercase and uppercase letters have different character codes. If the comparison you want to make is independent of case, then you will need to convert all of the string, or more precisely a variable which holds that value, to a case which is the same as the case of the variable with which it is being compared.<br /> <br /> 6<br /> <br /> 238<br /> <br /> Day 6<br /> <br /> Listing 6.15 shows a simple string comparison.<br /> <br /> INPUT<br /> <br /> LISTING 6.15<br /> <br /> Carrying Out a Case-Insensitive String Comparison Using the<br /> <br /> toUpperCase() Method of the String Object (ToUpperCase.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>String comparisons</title> <script type=”text/javascript” language=”javascript”> <!-- // onload = function(){ var firstString = new String(“mary Had A litTle lamb”); var secondString = new String(“Mary had a little lamb”); document.write(“<h3>Direct comparison of the two strings</h3>”); var comparison = (firstString==secondString); document.write(“Are the strings equal?: “ + comparison); document.write(“<h3>Comparison after conversion to upper case</h3>”); var firstUpperString = firstString.toUpperCase(); var secondUpperString = secondString.toUpperCase(); document.write(“Are the strings equal?: “ + (firstUpperString==secondUpperString)); } // --> </script> </head> <body> </body> </html><br /> <br /> Extracting String Segments with charAt() The simplest of the methods used to find a sub-string in another string is the charAt() method. This method returns a single character taken from a point in another string that you specify. You specify this point by providing an index as a parameter to the charAt() method, for example: var str = “My string”; alert(str.charAt(0)); // alerts “M”<br /> <br /> Just as the elements of an array are numbered starting from 0, so are the characters in a string. Therefore, when you specify the 0th character as the parameter of the charAt() method, then it is the first character, the letter “M”, that this method returns.<br /> <br /> HTML Forms and the String Object<br /> <br /> 239<br /> <br /> When you know the string’s format, then the charAt() method can be the best method for extracting the characters you need.<br /> <br /> Searching for Sub-Strings with indexOf() For other occasions when you do not know the format of the string, then you will need to use other methods of the String object. With these you can look for certain strings without knowing at what point in another string they are located. The indexOf() method is used to find the position of the first occurrence of a sub-string. See Listing 6.16.<br /> <br /> INPUT<br /> <br /> LISTING 6.16<br /> <br /> Using the indexOf() Method (IndexOf.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the indexOf() method</title> <script type=”text/javascript” language=”javascript”> <!-- // var myString = “The cat sat on the mat”; var mySubstring = “sat”; var foundAtPosition; foundAtPosition = myString.indexOf(mySubstring,0); document.write(“<p>The string “ + mySubstring + “ was found at position “ + foundAtPosition); // --> </script> </head> <body> </body> </html><br /> <br /> The above code will return the index at which the first (or only) occurrence of the string “sat” occurred. The output of the document.write indicates that sat occurred at position 8, which is correct. It is the position of the first character of the substring being searched for that is found using the indexOf() method.<br /> <br /> ANALYSIS<br /> <br /> Replacing Sub-Strings with replace() We can replace sub-strings within a string. This is similar to a search and replace function in a word processor. For example, if you had a description of JavaScript but wanted to be sure that all mentions had the correct case, then you could use the replace()<br /> <br /> 6<br /> <br /> 240<br /> <br /> Day 6<br /> <br /> method to replace any incorrectly capitalized versions in the text. Listing 6.17 shows a simple example using the replace() method. LISTING 6.17<br /> <br /> INPUT<br /> <br /> Using the replace() Method of the String Object<br /> <br /> (Replace.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the replace() method of the String object</title> <script type=”text/javascript” language=”javascript”> <!-- // var originalString = “George Washington, President of the United States”; var replacedString = “Washington”; var replacementString = “Bush”; var newString = originalString.replace(replacedString, replacementString); document.write(“<h3>” + newString + “</h3”); // --> </script> </head> <body> </body> </html><br /> <br /> The replace() method of the originalString String object is used to substitute the string “Bush”, in the variable replacementString, for the string “Washington”, in the variable replacedString. On screen document.write() outputs the string “George Bush, President of the United States”.<br /> <br /> ANALYSIS<br /> <br /> Concatenating Strings with the concat() Method The concat() method will concatenate two strings together. To concatenate two strings you simply apply the method to a string and supply the string that is to be concatenated to the first string as a parameter to the concat() method. For example, to concatenate the string “Hello “ to the word “there”, we could use the concat() method, as shown here: “Hello “.concat(“there”);<br /> <br /> Of course this concatenation could be achieved just as easily using the + operator. In fact concatenating strings using the + operator is actually more concise, as shown by the following: “hello “ + “there”;<br /> <br /> HTML Forms and the String Object<br /> <br /> 241<br /> <br /> Because the + operator is more succinct than the concat() method used with a string literal, the concat() method is not widely used.<br /> <br /> Finding Sub-Strings with substr() In Chapter 4 you learned how to access the hash property of the location object by writing the following: var myHash = location.hash;<br /> <br /> The problem with this is that it returns the internal document link used preceded by the hash character (#). If you want to find out the name of the internal link then you would need to remove this character from the string that the hash property contains. To do this, use the substr() method of the String object, for example: var myAnchor = location.hash; myAnchor = myAnchor.substr(1);<br /> <br /> This will retrieve all the characters after the first character of the hash property’s value (that is the character at index zero) is removed, thereby removing the hash character. In Chapter 2, “Working with Data,” we talked about operator precedence and said that when a period character joins two items it means that the righthand one belongs to the left. We also said that when operators have the same precedence, then they are evaluated from left to right. Therefore we can actually write the above code all on one line like this: var myAnchor = location.hash.substr(1);<br /> <br /> The period and square brackets denote membership of one thing to another and have the highest operator precedence. Therefore the code the period joins will be evaluated first from left to right. The location.hash is evaluated and turns into a string. Only then is the next period applying the sub-string function evaluated because by then the location.hash has evaluated to its string value that is possible. Even if there was no content to the location object’s hash property using the substr() method, starting at the character 1 won’t cause an error. It will simply return an empty string. Having taken a brief look at some of the properties and methods of the String object, let’s look at how we can check for the presence of user input and prevent it from being sent to the server while it is incomplete.<br /> <br /> 6<br /> <br /> 242<br /> <br /> Day 6<br /> <br /> Checking User Input So far we have only looked at using HTML forms as a convenient means of letting a user interact with scripts on a page. Another common use is to use JavaScript as an aid to forms when used for their original purpose: collecting input from a user and sending it to the server. In this section we will look at how you can check user input (for completeness) and in Chapter 13, “Regular Expressions Make It Easier,” we will look at using regular expressions to check that user input matches the desired input for a field. When we talk about checking user input, we mean checking it for completeness before the data that a user has entered into a form is sent to the server. The first thing to say about pre-validation using JavaScript is that it should not be used as the only means of checking that the data to be sent is acceptable. Some Internet users use browsers that do not, or cannot, interpret JavaScript. If this is the case, then the form data will be sent to the server without any checks having been performed. If no validation is carried out on the server, then major errors or even security breaches could potentially occur. The aim of using JavaScript to check user input is to speed up the user’s browsing experience. When data is sent to the server there is the risk that, if not filled out properly, the data may have to be returned to the user for corrections. The time taken for the data to be passed through a modem, validated, and returned with instructions to make changes can be considerable and a frustration to the visitors to your site. Using JavaScript to prevalidate a form before it is sent will, for most users (for example, those with browsers that are JavaScript capable), mean almost instantaneous checking of data. When checking forms there is one event handler that you need to know about. This is the event handler. If your validation is to stop the form from submitting if the form is incocompletely filled out, then you need to use this event handler to do that. (We will look in Chapter 13, “Regular Expressions Make It Easier,” at how we can ensure that valid information is entered in each part of the form before it is submitted to the server.) More importantly, unlike earlier examples in the chapter, we want to ensure that the form is not submitted to the server until the form is correctly completed.<br /> <br /> onSubmit<br /> <br /> Listing 6.18 is significantly longer than listings earlier in this chapter. If you take each part one at a time you should find it all understandable. Refer back to Figure 6.1 if you want to remind yourself how the form looks on screen. We will check that three parts of the form are filled in or we will not allow the data to be submitted to the server. The users have to fill in their names, indicate their gender and give information about delivery of the prize, which might be theirs for completing the survey.<br /> <br /> HTML Forms and the String Object<br /> <br /> INPUT<br /> <br /> 243<br /> <br /> LISTING 6.18<br /> <br /> Ensuring That the User Has Entered Data Before the Data Is Sent to the Server (SimpleFormValidated.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”> <html> <head> <title>XMML.com - Online Survey</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsFormValid(){ var IsFormValid = false; if (document.MyForm.YourName.value!==”” && evalRadio() && document.MyForm.Comments.value!==””) { IsFormValid = true; } if (IsFormValid==false) DisplayErrors(); return IsFormValid; } // end IsFormValid() function function evalRadio() { var group = document.MyForm.Gender; for (var i=0; i<group.length; i++) { if (group[i].checked) break; } if (i==group.length) return false; else {return true; } // end else } // end evalRadio() function function DisplayErrors(){ if (document.MyForm.YourName.value==””) alert(“Please enter your name”); if (evalRadio()==false) alert(“Please check the radio button\nfor your gender”); if (document.MyForm.Comments.value==””) alert(“Please be sure to tell us the delivery address \nfor your free gift”); } // end DisplayErrors() function; // --> </script> </head> <body> <form action=”http://www.XMML.com” method=”POST” name=”MyForm” onsubmit=”return IsFormValid()”> <table width=”600”> <tr><th colspan=”3” align=”center”>XMML.com - Online Survey<br /><br /></th><br /> <br /> 6<br /> <br /> 244<br /> <br /> Day 6<br /> <br /> LISTING 6.18<br /> <br /> continued<br /> <br /> </tr> <tr> <td>Your Name:</td> <td> </td> <td><input type=”text” name=”YourName”/></td></tr> <tr> <td>Your Gender:</td> <td> </td> <td> <input type=”radio” name=”Gender” value=”Male”/>Male<br /> <input type=”radio” name=”Gender” value=”Female”/>Female<br /> </td> </tr> <tr><td>Which of our consultancy <br />services are you interested in?</td> <td align=”right”>  </td> <td> <input type=”checkbox” name=”XML”/> XML<br /> <input type=”checkbox” name=”XSLT”/> XSLT<br /> <input type=”checkbox” name=”SVG”/> SVG<br /> <input type=”checkbox” name=”XSL-FO”/> XSL-FO<br /> <input type=”checkbox” name=”XForms”/> XForms<br /> </td> </tr> <tr> <td>Which free gift would you prefer for filling out this survey?</td> <td> </td> <td> <select name=”FreeGift”> <option value=”Choice1”>Fresh Air</option> <option value=”Choice2”>A long life</option> <option value=”Choice3”>Contentment</option> </select> </tr> <tr> <td>Enter your comments in<br />the text box </td> <td> </td> <td><textarea name=”Comments” rows=”5” cols=”50”/></td> </tr> <tr> <td> </td> <td> </td> <td><br /><input type=”submit” value=”Send Form”/></td> </tr> </table> </form> </body> </html><br /> <br /> HTML Forms and the String Object<br /> <br /> 245<br /> <br /> The way that JavaScript works revolves around the onsubmit attribute of the <form> element. When the form is submitted, by clicking the “Send Form” button, the IsFormValid() function is called. The “return IsFormValid()” syntax for the onsubmit attribute ensures that it is the value (“true” or “false”) which is returned from the IsFormValid() function, which determines whether or not the form is submitted. If IsFormValid()returns false the form is not submitted.<br /> <br /> ANALYSIS<br /> <br /> Within the IsFormValid() function the variable IsFormValid is assigned the value false. Then the if statement applies three tests all of which have to be true for the following statement IsFormValid = true;<br /> <br /> to be executed. If any of the three tests are false, then the DisplayErrors() function is called. Three if statements within the DisplayErrors() function control which alert boxes display. If there are no errors, then the IsFormValid variable is set to “true” and the value “true” is returned in response to the onsubmit attribute. Then, and only then, can the form be submitted to a server. The tests we have applied to the text field, the radio buttons, and the text area are very simple ones—we have only checked to see that some data has been entered and that none of the three essential fields has been left blank. In other circumstances, you may want to check for the length of number (when checking credit cards, for example), whether a value entered is a valid date, and so forth. In production forms you may want to carry out more sophisticated checks on data. If you expect, for example, that a valid date has to be entered for a particular piece of data, then you would check if it was a valid JavaScript date.<br /> <br /> Summary In this chapter we have introduced you to the use of JavaScript with the Form object and form elements. You have been shown how to check values entered by the user, to assist the user by applying focus to a field you want to be filled in first, and how to prevent data from being sent to the server for processing while it is incomplete. In addition you have been introduced to some of the properties and methods of the String object.<br /> <br /> 6<br /> <br /> 246<br /> <br /> Day 6<br /> <br /> Workshop In this workshop we will review what you have learned about forms in this chapter.<br /> <br /> Q&A Q. Do forms have to be embedded in tables to be displayed on screen? A. No. You can embed forms in an HTML/XHTML Web page without the use of tables. However, using tables makes it possible to line up parts of the form to achieve an attractive presentation. An alternative approach is to use CSS for positioning. Q. Can I use more than one form on the same HTML/XHTML Web page? A. Yes, you can. However you will need to think carefully about how data is to be submitted to the server for processing. For instance, you may want to include a submit button only on the second form. Q. Is it possible to use patterns of characters to search for sub-strings? A. Yes, JavaScript has a RegExp object that allows you to use regular expressions. Regular expressions make it possible to search strings for patterns of characters in addition to searching them for literal sub-strings. In Chapter 13, “Regular Expressions Make It Easier,” we will examine how to use regular expressions in JavaScript. Q. Are password fields on forms totally secure? A. No. A password field keeps someone from looking over your shoulder seeing on screen the password characters you have typed, but when your password is sent to the server it is not encrypted or disguised in any way.<br /> <br /> Quiz 1. List the form elements into which you can type text. 2. How do you create a drop-down menu in a form?<br /> <br /> Quiz Answers 1. There are three form elements into which you can type text—the text field, the text area, and a password field. Of course, with the password field you see only an asterisk for each character you type in. 2. The <select> element with nested <option> elements lets you create a drop-down menu.<br /> <br /> HTML Forms and the String Object<br /> <br /> 247<br /> <br /> Exercises 1. Write a script to check for the substring “man” in the string “A manual manually for a manipulative manager”.<br /> <br /> man wrote a<br /> <br /> 2. Create a form that asks for a name and credit card information. Write a script that checks that each field has been filled in before the data is sent to the server.<br /> <br /> 6<br /> <br /> WEEK 1<br /> <br /> DAY<br /> <br /> 7<br /> <br /> Numbers and Math An essential part of any programming language is the set of tools for working with numbers, and JavaScript has many such tools. If you aren’t particularly enthusiastic about the prospect of creating JavaScript applications that are heavily math-oriented then don’t worry. The Core JavaScript Number and Math objects provide some powerful mathematical tools to make many of the processes and calculations much easier. As you will see, even when our specific requirements are not covered directly by what JavaScript provides, we can frequently create our own tools with a little bit of ingenuity. The number of possible JavaScript uses for numbers is enormous. Unless you are particularly interested in math, then you will find that JavaScript provides many mathematical techniques that you may never use. However, in common with all worthwhile programming languages, JavaScript needs to provide powerful and flexible facilities that will allow interested programmers to manipulate numbers. This chapter will introduce you to some foundational numeric techniques that you can build on if you are interested in this topic. However, we can’t look at all the possible numerical techniques for which you could use JavaScript.<br /> <br /> 250<br /> <br /> Day 7<br /> <br /> This chapter will introduce you to • The Number object and its properties and methods • The Math object and its properties and methods • Examples of carrying out calculations using the Math object<br /> <br /> The Number Object In practice the Number object is seldom used directly by most JavaScript scripters. Usually you only use its methods, which can be very helpful—especially those methods that have been recently introduced in JavaScript 1.5. However, before we examine the Number object’s properties and methods, let’s quickly take a general look at how numbers are treated in JavaScript.<br /> <br /> Numbers in JavaScript For integers, the numbers that JavaScript outputs seem to be spot on. However for floating-point numbers the situation isn’t quite so simple. By default both Internet Explorer and Netscape Navigator will return floating-point numbers rounded to 16 or 17 significant figures. Unfortunately because JavaScript uses the computer’s floating-point math (which isn’t always accurate to this precision) the results of JavaScript’s floating-point calculations are not always entirely accurate. For example, if we divide 10.0 by 3.0 the answer given will be 3.3333333333333335, as running Listing 7.1 will show.<br /> <br /> INPUT<br /> <br /> LISTING 7.1<br /> <br /> Exploring the Precision of Numbers (NumberPrecison.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Precision of numbers</title> <script type=”text/javascript” language=”javascript”> <!-- // function Calculate(){ var n = 10.0 / 3.0; document.write(“10.0 divided by 3.0 is “ + n); } // --> </script> </head> <body onload=”Calculate()”> </body> </html><br /> <br /> Numbers and Math<br /> <br /> 251<br /> <br /> Most of the time when a piece of arithmetic is expected to return a floating-point number this small glitch at the end of the fractional part isn’t too much of a problem. But when numbers should return an integer or are to be output to the screen for a user to see, then small discrepancies can be annoying. If you modify Listing 7.1 so that 0.3 is divided by 0.1, then the answer returned is 2.9999999999999996, which is correct when rounded but may very likely not be what was expected by the user. Fortunately, for situations such as these, we can remedy this problem to a certain extent as far as the display of numbers is concerned. Listing 7.2 shows how you might handle the situation of dividing 10.0 by 3.0 to a selected number of decimal places.<br /> <br /> INPUT<br /> <br /> LISTING 7.2 Controlling the Output Format of Numbers (FormatNumber.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Formatting numbers to a chosen number of decimal places</title> <script type=”text/javascript” language=”javascript”> <!-- // function Calculate(){ var entry; var first; var second; var numPlaces; alert(“Divide a larger number by a smaller number\nto a chosen number of decimal places\nClick to continue”); entry = prompt(“Enter first number: “, “Enter number here”); first = Number(entry); entry = prompt(“Enter second number: “, “Enter number here”); second = Number(entry); entry = prompt(“Enter number of decimal places. “, “Enter number here”); numPlaces = Number(entry); var n = first / second; document.write(first + “ divided by “ + second + “ to “ + numPlaces + “ decimal places is “ + formatNumber(n,numPlaces)); } function formatNumber(theNum, numDecPlaces) { var num = new String(); num = “” + theNum; var pos = 0; count = 0; while (num.substring(pos-1,pos)!== “.”) { pos += 1 ;<br /> <br /> 7<br /> <br /> 252<br /> <br /> Day 7<br /> <br /> LISTING 7.2<br /> <br /> continued<br /> <br /> count += 1; } //end while while (pos < (count+numDecPlaces)){ pos +=1; } // end while return num.substring(0,pos); } // --> </script> </head> <body onload=”Calculate()”> </body> </html><br /> <br /> Let’s look at how the code works. On the <body> element there is an onload attribute which calls the Calculate() function. Within the Calculate() function we use an alert() box to inform the user what the page will do. We use three prompt() boxes to gather three numbers chosen by the user. The entry variable is used to hold the “number” chosen by the user in each prompt() box but, of course, the “number” entered is actually held as a string. Therefore, in order to use the “numbers” entered for calculation, we must convert each of those strings to a number, using code like this:<br /> <br /> ANALYSIS<br /> <br /> first = Number(entry);<br /> <br /> We divide the first number, stored in the first variable, by the second, stored in the second variable, and store the result of the division in the n variable.<br /> <br /> Caution<br /> <br /> When you enter a numerical value in a prompt() box the value is held as a string. If you attempt to perform calculations then you can expect an error since the value on which a calculation is being attempted is a string. To avoid such errors you need to convert the numeric-looking string to a number using the Number() function.<br /> <br /> We used document.write() to output the result of the calculation. Within the document.write() we called the formatNumber() function passing two parameters—the result of the calculation in the variable n and the desired number of decimal places in the variable numPlaces.<br /> <br /> Numbers and Math<br /> <br /> 253<br /> <br /> We needed to convert the string entries entered into the prompt() boxes in the Calculate() function to numbers; however, in the formatNumber() function, we need to do the opposite and convert the result of the calculation to a string, held in the num variable, like so: var num = new String(); num = “” + theNum;<br /> <br /> The theNum variable is the parameter passed into the formatNumber() function. We then used the substring() method of the String object (to which you were introduced in Chapter 6, “HTML Forms and the String Object”) to work through the string variable num character by character. While we haven’t reached the decimal point in that string, we just will carry on looping round a while loop, increasing a counter as we go. Once we do reach the decimal point, then we loop again until we have moved the required number of positions through the string variable; then we simply return the num string variable to the document.write() within the Calculate() function. The result of dividing 10.0 by 3.0 to 4 decimal places is shown in Figure 7.1.<br /> <br /> OUTPUT FIGURE 7.1 The result of the output from Listing 7.2 when dividing 10.0 by 3.0 to 4 decimal places.<br /> <br /> Controlling the format when you expect integer numbers is easier, if you are sure that the result will be an integer. Listing 7.3 is an example of how you can round numbers to achieve an integer result.<br /> <br /> INPUT<br /> <br /> LISTING 7.3 Using the round() Function of the Math Object to Output Numbers as Integers (OutputInteger.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”><br /> <br /> 7<br /> <br /> 254<br /> <br /> Day 7<br /> <br /> LISTING 7.3<br /> <br /> continued<br /> <br /> <html> <head> <title>Rounding a number to an integer</title> <script type=”text/javascript” language=”javascript”> <!-- // function IntegerCalculate(){ var num1 = 0.3; var num2 = 0.1; var result ; result = num1 / num2; document.write(num1 + “ divided by “ + num2 + “ expressed as a whole number is “ + Math.round(result)); } // --> </script> </head> <body onload=”IntegerCalculate()”> </body> </html><br /> <br /> The round() method of the Math object is used to convert the result variable to an integer number. Of course, you will only want to use the round() method when you either confidently expect the answer to be a whole number or can accept an integer as an adequate approximation of a floating-point value.<br /> <br /> ANALYSIS<br /> <br /> It is also worth noting before we go on that JavaScript allows you to enter exponential numbers using the letter “e” (or uppercase “E” if you prefer). As in the following: 1e6 1E6 1e+6 1e-6<br /> <br /> // // // //<br /> <br /> equivalent equivalent equivalent equivalent<br /> <br /> to to to to<br /> <br /> 1,000,000 1,000,000 1,000,000 0.000001<br /> <br /> Notice that when the exponent is to a positive power, then the plus sign does not need to be included. But naturally for a number to a negative exponent, the minus sign must be present.<br /> <br /> Note<br /> <br /> When using the exponent “E” or “e”, the JavaScript interpreter will accept either upper or lower case, one of the few situations in which a JavaScript interpreter will treat upper and lower case the same.<br /> <br /> Let’s return to the Number object and take a look at its properties.<br /> <br /> Numbers and Math<br /> <br /> 255<br /> <br /> Properties of the Number object The Number object has several built-in properties. As with other objects, they include the constructor and prototype properties that we will look at later in the book. The other properties are shown below: •<br /> <br /> MAX_VALUE<br /> <br /> •<br /> <br /> MIN_VALUE<br /> <br /> •<br /> <br /> NaN<br /> <br /> •<br /> <br /> NEGATIVE_INFINITY<br /> <br /> •<br /> <br /> POSITIVE_INFINITY<br /> <br /> We have already met the NaN property earlier in the book. Hopefully you remember that it stands for Not a Number and that it is the inevitable result of trying to perform an arithmetic process where one or more of the pieces of data is not numerical. Listing 7.4 shows how you can test whether or not a user enters a string that is numeric.<br /> <br /> INPUT<br /> <br /> LISTING 7.4<br /> <br /> Testing Whether or Not a String Entered Can Be Converted to a Valid Number (NumberNaN.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>The NaN property of the Number object</title> <script type=”text/javascript” language=”javascript”> <!-- // function Calculate(){ var entry; var number = new Number(); alert(“This page will test whether you enter a \nnumber or a non-numeric string.\nClick to continue”); entry = prompt(“Enter a number or string: “, “Enter number or string here”); number = Number(entry); if (isNaN(number)){ document.writeln(“<p>You entered: “ + entry+ “</p>”); document.write(“<p>It was not a number</p>”);} else{ document.writeln(“<p>You entered: “ + entry + “</p>”); document.write(“<p>It was a number.</p>”); } // end else } // end function Calculate() // --> </script><br /> <br /> 7<br /> <br /> 256<br /> <br /> Day 7<br /> <br /> LISTING 7.4<br /> <br /> continued<br /> <br /> </head> <body onload=”Calculate()”> </body> </html><br /> <br /> When you load the page the user is presented with an informational alert() box. Then, in a prompt() box, the user is asked to enter either a number or a string. The Number() function is then applied to the string—held in the entry variable— that the user had entered. Finally the isNaN() function is used to test whether or not a valid number resulted from applying the Number() function to the entry variable. If no valid number resulted, then the user is shown the string they entered and informed that it was not a number. If, however, the entry variable can be converted into a valid number, then the user is reminded of her entry into the prompt() box and told that it was a number. Figure 7.2 shows the result when the non-numeric string “abc” was entered into the prompt box.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 7.2 The result of applying the isNaN() function to test a nonnumeric value.<br /> <br /> Tip<br /> <br /> The isNaN() function is the only way to test whether or not a value matches NaN. So, if you want to test whether a value is or is not a number remember to use the isNaN() function. Do not attempt to test the NaN property for equality for example by using if(x == NaN). It doesn’t work.<br /> <br /> The other properties of the Number object reveal key JavaScript values. Note that the name of each property is written completely in uppercase letters. If you remember what we said about naming using uppercase letters in Chapter 2, “Working with Data,” you may realize that this shows that they are constants. Therefore we cannot change the values they contain because they are defined by the JavaScript interpreter and/or the environment in which it is operating.<br /> <br /> Numbers and Math<br /> <br /> MAX_VALUE<br /> <br /> 257<br /> <br /> and MIN_VALUE<br /> <br /> The MAX_VALUE and MIN_VALUE properties contain the value of the absolute maximum and minimum numbers that the JavaScript interpreter will accept before it treats them simply as Infinity or zero respectively. Listing 7.5 displays the maximum and minimum values for you. LISTING 7.5<br /> <br /> Displaying the MAX_VALUE and MIN_VALUE Properties of the Number Object<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Displaying Maximum and Minimum Values</title> <script type=”text/javascript” language=”javascript”> <!-- // function ShowLimits(){ var maxAndMin = “Max Number: “ + Number.MAX_VALUE + “\n”; maxAndMin += “Min Number: “ + Number.MIN_VALUE; alert(maxAndMin); } // --> </script> </head> <body onload=”ShowLimits()”> </body> </html><br /> <br /> The result you see may vary according to the platform you use. Figure 7.3 shows the alert box as it appears in Netscape 6.<br /> <br /> OUTPUT FIGURE 7.3 The maximum and minimum values of the Number object displayed in a Netscape 6 alert box.<br /> <br /> 7<br /> <br /> 258<br /> <br /> Day 7<br /> <br /> You may remember that we also mentioned these values in passing in Chapter 1, “Getting the Basics Right,” as exponentials of the number 2. To do this we used these values together with the toString() function to turn them into binary form. For example to find the maximum value you could use the following: var x = Number.MAX_VALUE; x = x.toString(2); alert(x);<br /> <br /> NEGATIVE_INFINITY<br /> <br /> and POSITIVE_INFINITY<br /> <br /> The NEGATIVE_INFINITY and POSITIVE_INFINITY properties of the Number object are also JavaScript constants. There is no real need to use these properties because if you ever need to obtain the values infinity or negative infinity, you can simply write the following: Infinity -Infinity<br /> <br /> rather than navigating to the values through the Number object like this: Number.POSITIVE_INFINITY Number.NEGATIVE_INFINITY<br /> <br /> You could also simply divide 1 or -1 by 0 to get the same effect, but it is recommended that you not do so because using the values Infinity and -Infinity makes your scripts clearer. Let’s move on and look at the methods of the Number object.<br /> <br /> Methods of the Number Object The Number object’s methods are significantly more useful to us than its properties. This is especially so for the methods that were added to the language after the release of ECMAScript Edition 3. The Number object’s methods are listed below: •<br /> <br /> toExponential()<br /> <br /> •<br /> <br /> toFixed()<br /> <br /> •<br /> <br /> toLocaleString()<br /> <br /> •<br /> <br /> toString()<br /> <br /> •<br /> <br /> toPrecision()<br /> <br /> •<br /> <br /> valueOf()<br /> <br /> Numbers and Math<br /> <br /> 259<br /> <br /> You have already seen the toString() function at work in Listing 7.1. Because the toString() function is a global function it is available as a method of all Core JavaScript objects—including the Number object. When it is used with numbers it simply returns the number converted to the data type string. The toExponential(), toFixed(), toLocaleString(), and toPrecision() methods are all new to Internet Explorer 5.5 and Netscape Navigator 6. All of these methods help with number formatting so that the numbers can be displayed in the way that best suits your requirements or the requirements of visitors to your Web sites. First let’s look at the toExponential() method.<br /> <br /> Using the toExponential() Method As mentioned previously, a JavaScript interpreter always converts numbers to their decimal form before it displays them. To force the JavaScript interpreter to display numbers in a base other than base 10, you can use the toString() method. The toString() method works because it outputs the number as the data type string. Therefore JavaScript ignores the order of the characters and doesn’t try to convert them to a decimal value. The same idea (conversion of the data type to string) can be used to help output numbers in other formats that JavaScript wouldn’t otherwise allow. JavaScript will only display numbers between 1000000000000000 (1e15) and 0.000001 in the standard non-exponential format. Even if you input them in exponential form they are immediately converted to plain numbers. Internet Explorer 5.5+ and Netscape Navigator 6+ allow you to override these rules and display numbers within the range in exponential form. The two methods that provide you with a means to do this are toExponential() and toPrecision(). (1e-5)<br /> <br /> The toExponential() method simply displays the numbers it is given in exponential form (as a string). There is nothing special about it other than that. It is applied to a data container using the dot notation, like so: var myNum = 1234; myNum = myNum.toExponential();<br /> <br /> // returns 1.234e+3<br /> <br /> The method can also be given an optional parameter to control how many decimal places are returned. This can be especially useful for anyone wanting to display numbers in scientific notation. For example, to display the previous number to 3 decimal places, you would simply write: var myNum = 1234; myNum = myNum.toExponential(4); myNum = myNum.toExponential(2);<br /> <br /> 7 // returns 1.2340e+3 // returns 1.23e+3<br /> <br /> 260<br /> <br /> Day 7<br /> <br /> Listing 7.6 takes a number entered by the user and displays it with from zero to four decimal places.<br /> <br /> INPUT<br /> <br /> LISTING 7.6<br /> <br /> Using the toExponential() Method to Display a User-Entered Number as an Exponential (toExponential.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Demonstrating the toExponential() method</title> <script type=”text/javascript” language=”javascript”> <!-- // function CreateExponentials(){ var entry1 ; entry1 = prompt(“Enter a floating point number”, “Enter the number here”); number = new Number(); number = Number(entry1); for (var i=0; i<5; i++){ document.write(“<p>The number entered to “ + i + “ decimal places is “ + number.toExponential(i) + “</p><br/>”); } } // end function CreateExponentials() // --> </script> </head> <body onload=”CreateExponentials()”> <p>This listing demonstrates the use of the toExponential() method.</p><br /> </body> </html><br /> <br /> On page load the CreateExponentials() function is called. A prompt box is used to get a floating-point number from the user. Because the toExponential() method belongs to the Number object, you convert the string value entry1 to a numeric value stored in the number variable.<br /> <br /> ANALYSIS<br /> <br /> The listing simply loops through a for loop displaying the value entered by the user with the number of decimal places indicated by the loop counter variable, i, of the for loop.<br /> <br /> The toPrecision() Method Although you may not have much need to display your numbers in exponential notation you will more than likely want to control their accuracy at some point or another. To do this you have two options. You can either decide on a number of significant figures, or you can decide on a number of decimal points. To choose the accuracy in terms of the number of significant figures, you use the toPrecision() method. The method is<br /> <br /> Numbers and Math<br /> <br /> 261<br /> <br /> applied with the dot notation and the number of significant figures is controlled by specifying the number as a parameter of the method. To see this at work, look at the following lines: var var var var var<br /> <br /> myNum to8SF to6SF to4SF to2SF<br /> <br /> = = = = =<br /> <br /> 1234.56; myNum.toPrecision(8) myNum.toPrecision(6) myNum.toPrecision(4) myNum.toPrecision(2)<br /> <br /> // // // //<br /> <br /> results results results results<br /> <br /> in in in in<br /> <br /> 1234.5600 1234.56 1234 1.2e+3<br /> <br /> Listing 7.7 shows a full example.<br /> <br /> INPUT<br /> <br /> LISTING 7.7<br /> <br /> Using the toPrecision() Method to Control Display of a Number (toPrecision.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Demonstrating the toPrecision() method</title> <script type=”text/javascript” language=”javascript”> <!-- // function DisplayPrecision(){ var entry1 ; entry1 = prompt(“Enter a floating point number”, “Enter the number here”); number = new Number(); number = Number(entry1); for (var i=1; i<8; i++){ document.write(“<p>The number entered to “ + i + “ significant figures is “ + number.toPrecision(i) + “</p><br/>”); } } // end function DisplayPrecision() // --> </script> </head> <body onload=”DisplayPrecision()”> <p>This listing demonstrates the use of the toPrecision() method.</p><br /> </body> </html><br /> <br /> ANALYSIS<br /> <br /> Notice that in the above example that we start the i variable for the for loop at 1, since a value of zero makes no sense for the number of significant figures.<br /> <br /> When the number of significant figures specified is greater than the number itself, then the method adds on zeros. When the number of significant figures you specify is greater than the whole part of the number, then the number is returned in the standard form; but<br /> <br /> 7<br /> <br /> 262<br /> <br /> Day 7<br /> <br /> when the significant figures specified is smaller than the whole part, it’s turned into exponential form. Effectively it then acts as the toExponential() method does when you pass it a number as a parameter. If this isn’t what you want, then you can simply apply the parseFloat() function to the result. By turning the result into a number then, as long as the value is within the range specified earlier, the number will be displayed in normal form. For example to do this for the last example you could write the following: var myNum = 1234.56; var to2SF = myNum.toPrecision(2); var to2SF = parseFloat(to2SF); // results in 1200<br /> <br /> Alternatively as a shortcut, you could simply multiply the result by one, although the results of the toPrecision() method are in string form. Remember that JavaScript converts the operands of operators to the data type that make most sense for the operation. The only data type that makes sense for multiplication is the type number so you could have written it like so: var myNum = 1234.56; var to2SF = myNum.toPrecision(2) * 1; // results in 1200<br /> <br /> The toPrecision() method may seem to offer a way to force JavaScript to display to a greater accuracy than its standard 16/17 figures. After all, can’t we just use the method to specify a number greater than this? Unfortunately, although it can be used to show a greater number of figures, these figures are not accurate. To illustrate this let’s use a 20-digit number with figures that iterate from 1 through 0 twice: var myNum = 12345678901234567890; myNum = myNum.toPrecision(20); alert(myNum);<br /> <br /> Normally this number would be truncated to 17 characters and shown in exponential form. Using the toPrecision() method like this in Internet Explorer 5.5 makes the browser display 20 characters. However the characters after the second seven characters are simply replaced by zeros. Netscape Navigator 6 on the other hand is inconsistent with the digits it displays after the 17th digit of a long number. In the case above, it replaces the digits 890 that we might hope for with the digits 168. The fact is that JavaScript doesn’t handle numbers of such length with accuracy.<br /> <br /> The toFixed() Method Of the four new methods of the Number object, the one you will probably have the most use for is the toFixed() method. Rather than returning a value to a fixed number of significant figures as the toPrecision() method does, it returns numbers to a fixed number of decimal places.<br /> <br /> Numbers and Math<br /> <br /> 263<br /> <br /> One aspect of JavaScript that is most frustrating to those who want to use it to deal with money is its handling of numbers. JavaScript will remove any trailing zeros after a decimal point. Therefore if we were to total up the cost of several items using JavaScript, then output it to a Web page without any further processing, we would not know whether it would display correctly. For example, look at the following example of code: var costOfItem1 = 9.10; var costOfItem2 = 5.60; var totalCost = costOfItem1 + costOfItem2;<br /> <br /> // returns 14.7<br /> <br /> Clearly, for example, 14.7 would not be acceptable as a currency value for any Web site that might use some sort of JavaScript enhancement to calculate the cost of a visitor’s potential shopping cart purchase. There is a workaround for this which you will see shortly, but in the future once post-version 6 browsers become the norm, the toFixed() method should make this redundant. Let’s look at some examples: var var var var var var<br /> <br /> myNum to6DP to4DP to2DP to0DP toN2DP<br /> <br /> = = = = = =<br /> <br /> 1234.5678; myNum.toFixed(6) myNum.toFixed(4) myNum.toFixed(2) myNum.toFixed(0) myNum.toFixed(-2)<br /> <br /> // // // // //<br /> <br /> results results results results results<br /> <br /> in in in in in<br /> <br /> 1234.567800 1234.5678 1234.56 1234 1200<br /> <br /> As you can see the method does exactly as you would expect. The possible exception to this is the last example where the parameter was set to -2 decimal points. This rounded the number to the hundreds mark. However at the time of writing, this only worked in Netscape Navigator 6. In the previous example you could display the total cost of the items as currency by writing the following: var var var var<br /> <br /> costOfItem1 costOfItem2 totalCost = totalCost =<br /> <br /> = 9.10; = 5.60; costOfItem1 + costOfItem2; totalCost.toFixed(2);<br /> <br /> // returns 14.7 // returns 14.70<br /> <br /> You could if you wanted to then concatenate this with a dollar symbol or a symbol of another currency as appropriate for the Web site. Note that if the toFixed() method is used with a very large number such as an exponential, the result will still be displayed in standard notation. For example, as that shown here: var myNum = 1e30; myNum = myNum.toFixed(3); alert(myNum); // alerts 1000000000000000000000000000000.000<br /> <br /> 7<br /> <br /> 264<br /> <br /> Day 7<br /> <br /> There are occasions when this could be useful. For example if you don’t want to output a large number in exponential form, you could use this method with the parameter set to zero.<br /> <br /> The toLocaleString() Method Although the toFixed()method can be used for formatting numbers as currency, it is the toLocaleString() method that is specifically intended for this job. It uses the system language of the computer it is running on to format a number to the currency notation of that language. Let’s look at an example that illustrates this. Listing 7.8 will allow a user to enter a floating-point number that is displayed according to the browser settings.<br /> <br /> INPUT<br /> <br /> LISTING 7.8 Using the toLocaleString() Method to Demonstrate the Current Browser Language Settings (toLocaleString.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Demonstrating the toLocaleString() method</title> <script type=”text/javascript” language=”javascript”> <!-- // function DisplayLocale(){ var entry1 ; entry1 = prompt(“Enter a floating point number”, “Enter the number here”); number = new Number(); number = Number(entry1); document.write(“<p>The number entered by the user expressed for the locale is “ + number.toLocaleString() + “</p><br/>”); } // end function DisplayLocale() // --> </script> </head> <body onload=”DisplayLocale()”> <p>This listing demonstrates the use of the toPrecision() method.</p><br /> </body> </html><br /> <br /> Again, remember that the toLocaleString() method belongs to the Number object and that a prompt box accepts a string value. In the United States or in the United Kingdom, Listing 7.8 will result in the display shown in Figure 7.4 when the number 1234.56 is entered by a user.<br /> <br /> Numbers and Math<br /> <br /> 265<br /> <br /> OUTPUT FIGURE 7.4 Display using toLocaleString() as it would appear in the United States or United Kingdom.<br /> <br /> ANALYSIS As you can see, it removes the extraneous digits after the decimal point and inserts commas between each triplet of the whole part of the number.<br /> <br /> However, other countries don’t format numbers in this way. So if the system’s language setting is German, for example, then the listing will format it differently, as shown in Figure 7.5.<br /> <br /> Tip<br /> <br /> To alter the system language settings in Windows 98, for example, choose Start, Control Panel, Regional Settings. On the Regional Settings tab choose, for example, German (Standard).<br /> <br /> OUTPUT FIGURE 7.5 Display using as it appears when the browser language is set to German in Internet Explorer.<br /> <br /> toLocaleString()<br /> <br /> ANALYSIS This time it has used a comma as the decimal point and periods to separate each triplet, as is the standard German numerical notation.<br /> <br /> Caution<br /> <br /> At the time of writing although the toLocaleString() method was recognized by Netscape 6, it did not seem to have any effect on the numbers to which it was applied.<br /> <br /> 7<br /> <br /> 266<br /> <br /> Day 7<br /> <br /> Many European languages use a format similar to that displayed in Figure 7.5 for the German language. Remember from the previous chapter that we stated that the toLocaleString() method is also applicable to arrays. If you have gathered monetary data and stored it in a string, you do not need to apply the toLocaleString() method to each element of the array in turn. You simply apply it to the array as a whole.<br /> <br /> Caution<br /> <br /> It is important to remember that the toExponential(), toFixed(), toLocaleString(), and toPrecision() methods all return their results as strings. If the number is then to be used in an addition operation, you must convert it to the data type number; otherwise the + operator will concatenate strings, not perform an addition calculation.<br /> <br /> The valueOf() method The valueOf() method of a Number object returns the value of a Number object as a number datatype. If JavaScript had been created from the start as a fully object-oriented technology you might well have expected a Number object to possess a value property. The fact that it doesn’t is a reflection of the time in JavaScript’s history when it was much less object-oriented than JavaScript 1.5. Listing 7.9 shows an example using the Number object’s valueOf() method. LISTING 7.9<br /> <br /> Finding the Numeric Value of a Number Object Using the valueOf() Method<br /> <br /> (valueOf.htm) <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Demonstrating the valueOf() method</title> <script type=”text/javascript” language=”javascript”> <!-- // function DisplayValue(){ var entry1 ; entry1 = prompt(“Enter a floating point number”, “Enter the number here”); var number = new Number(); number = Number(entry1); if (isNaN(number)){ document.write(“<p>You did not enter a valid number.<br /> Hit your browser’s Refresh button to try again.</p>”); }<br /> <br /> Numbers and Math<br /> <br /> LISTING 7.9<br /> <br /> 267<br /> <br /> continued<br /> <br /> else{ document.write(“<p>The value of the number entered is “ + number.valueOf() + “</p>”); } // end else } // end function DisplayValue() // --> </script> </head> <body onload=”DisplayValue()”> <p>This listing demonstrates the use of the valueOf() method.</p><br /> </body> </html><br /> <br /> The processing of an entered number seems to do very little, but as was the case in some examples earlier in this chapter, there is quite a bit of type casting going on. The string entered in the prompt box is assigned to the Number object in the variable number. Before using the document.write() method we use the isNaN() function to output an error message; if no valid number was entered and, if a valid number was entered into the prompt box, then we output the result returned by the valueOf() method of the Number object. Having looked at the properties and methods of the Number object, let’s move on and learn a little about the Math object.<br /> <br /> The Math Object You have already seen, and are hopefully now comfortable with, a lot of the mathematical operators that JavaScript provides. For much of the time those operators are perfectly adequate for the arithmetical processes that need to be carried out. But whenever more than the most basic of mathematical operations is needed, then we need something more specific or more powerful. JavaScript has its own Math object. The Math object provides some powerful tools and useful mathematical constants, which we can use in any mathematical operations.<br /> <br /> Predefined Properties The Math object’s properties are all constants so as you would expect they are read-only and can’t be changed. In line with traditional constant notation in JavaScript, the names of the properties are written completely in uppercase letters. Be careful to remember this, as writing the properties with one or more lowercase letters is a common mistake. See Table 7.1.<br /> <br /> 7<br /> <br /> 268<br /> <br /> Day 7<br /> <br /> TABLE 7.1<br /> <br /> The Mathematical Constants Provided in the Math Object’s Properties<br /> <br /> Property<br /> <br /> Value<br /> <br /> Description<br /> <br /> E<br /> <br /> 2.718281828459045<br /> <br /> Euler's constant<br /> <br /> LN2<br /> <br /> 0.6931471805599453<br /> <br /> Natural log of 2<br /> <br /> LN10<br /> <br /> 2.302585092994046<br /> <br /> Natural log of 10<br /> <br /> LOG2E<br /> <br /> 1.4426950408889634<br /> <br /> Log to base 2 of E<br /> <br /> LOG10E<br /> <br /> 0.4342944819032518<br /> <br /> Log to base 10 of E<br /> <br /> PI<br /> <br /> 3.141592653589793<br /> <br /> π<br /> <br /> SQRT1_2<br /> <br /> 0.7071067811865476<br /> <br /> Square root of 0.5<br /> <br /> SQRT2<br /> <br /> 1.4142135623730951<br /> <br /> Square root of 2<br /> <br /> INPUT<br /> <br /> LISTING 7.10<br /> <br /> The Properties of the Math Object (MathProperties.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>The Properties of the Math object</title> <script type=”text/javascript” language=”javascript”> <!-- // function DisplayMath(){ document.write(“<br/><p>The value of Euler’s constant <i>E</i> is: “ + Math.E + “</p>”); document.write(“<br/><p>The value of the natural Logarithm of 2 is: “ + Math.LN2 + “</p>”); document.write(“<br/><p>The value of the natural Logarithm of 10 is: “ + Math.LN10 + “</p>”); document.write(“<br/><p>The value of the Logarithm to base 2 of E is: “ + Math.LOG2E + “</p>”); document.write(“<br/><p>The value of the Logarithm to base 10 of E is: “ + Math.LOG10E + “</p>”); document.write(“<br/><p>The value of the constant PI is: “ + Math.PI + “</p>”); document.write(“<br/><p>The value of the square root of 1/2 is: “ + Math.SQRT1_2 + “</p>”); document.write(“<br/><p>The value of the square root of 2 is: “ + Math.SQRT2 + “</p>”); } // end function DisplayMath() // --> </script> </head> <body onload=”DisplayMath()”> </body> </html><br /> <br /> Numbers and Math<br /> <br /> 269<br /> <br /> As with other numbers, the values held by the Math object’s properties are accurate to approximately 16 significant figures. When using these properties remember that they must be referred to as properties of the Math object. Also take care to remember that the Math object starts with an uppercase “M”. If you only use the Math object occasionally, then both of these important details are easy to forget. Listing 7.11 shows an example of how to use the Math.PI property in calculating the area and circumference of a circle.<br /> <br /> INPUT<br /> <br /> LISTING 7.11<br /> <br /> Using Math.PI to Calculate the Area and Circumference of a Circle (FindArea.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Find the area and circumference of a circle</title> <script type=”text/javascript” language=”javascript”> <!-- // function FindArea(){ var entry1 ; entry1 = prompt(“Enter the radius of a circle”, “Enter the number here”); var number = new Number(); number = Number(entry1); if (isNaN(number)){ document.write(“<p>You did not enter a valid number.<br /> Hit your browser’s Refresh button to try again.</p>”); } else{ document.write(“<p>You entered a radius of “ + number + “</p>”); document.write(“<p>The area of the circle is “ + (number * number * Math.PI) + “</p>”); document.write(“<p>The circumference of the circle is “ + (2 * number * Math.PI) + “</p>”); } // end else } // end function FindArea() // --> </script> </head> <body onload=”FindArea()”> </body> </html><br /> <br /> 7<br /> <br /> 270<br /> <br /> Day 7<br /> <br /> We ask the user to input a chosen radius for the circle and, if a valid number has been entered, we calculate the area and circumference of the circle—each calculation uses Math.PI, as well as the document.write() method to output the results to screen. Let’s move on to look at the methods of the Math object.<br /> <br /> Methods of the Math Object No matter what you are using JavaScript for it is unlikely, unless you want to use it for ambitious mathematics, that you will find it lacking in terms of mathematical functions. The Math object has a large number of methods that cover many eventualities. See Table 7.2. TABLE 7.2<br /> <br /> The Methods of the JavaScript Math Object<br /> <br /> Method<br /> <br /> Description<br /> <br /> abs()<br /> <br /> Returns the absolute (positive) value of a number<br /> <br /> acos()<br /> <br /> Returns the arc cosine of a value<br /> <br /> asin()<br /> <br /> Returns the arc sine of a value<br /> <br /> atan()<br /> <br /> Returns the arc tangent of a value<br /> <br /> atan2()<br /> <br /> Returns the angle of two polar coordinates<br /> <br /> ceil()<br /> <br /> Rounds up a fractional number to the next integer<br /> <br /> cos()<br /> <br /> Returns the cosine of an angle<br /> <br /> exp()<br /> <br /> Returns Euler’s constant raised to the power of a number<br /> <br /> floor()<br /> <br /> Rounds down a fractional number to the next integer<br /> <br /> log()<br /> <br /> Returns the natural logarithm of a number<br /> <br /> max()<br /> <br /> Returns the maximum number from a comma separated list<br /> <br /> min()<br /> <br /> Returns the minimum number from a comma separated list<br /> <br /> pow()<br /> <br /> Returns one number raised to the power of another<br /> <br /> random()<br /> <br /> Generates a random number between 0 and 1<br /> <br /> round()<br /> <br /> Rounds a number to the nearest integer (rounding .5 up)<br /> <br /> sin()<br /> <br /> Returns the sine of an angle<br /> <br /> sqrt()<br /> <br /> Returns the square root of a number<br /> <br /> tan()<br /> <br /> Returns the tangent of an angle<br /> <br /> Numbers and Math<br /> <br /> Caution<br /> <br /> 271<br /> <br /> Remember that the properties of the Math object are expressed only in uppercase characters and the methods of the Math object are expressed in lowercase characters. Mixing the case of a property or method name can cause obscure errors, particularly in lengthy calculations.<br /> <br /> Many of these methods provide trigonometric functions. Maybe you already know that you will have a use for them, but if you don’t then don’t ignore them. They can be useful for all sorts of things such as creating some special dynamic HMTL effects. In the remainder of the chapter, we will look at some, but not all, of the Math object’s methods.<br /> <br /> Finding Absolute Values Using the abs() Method The simplest of the Math object’s methods is probably the abs() method. It finds the absolute value of the number it is applied to. The absolute value of a number is the difference from zero, and it is always expressed as a positive number, whether the original number was positive or negative. For example, as shown in the following: Math.abs( 1) Math.abs(-1)<br /> <br /> // returns 1 // returns 1<br /> <br /> Hopefully, from this example it isn’t too difficult to see that the absolute value of a number was the positive magnitude of that number.<br /> <br /> Finding Square Roots Using the sqrt() Method The sqrt() method is another method belonging to the Math object, and it returns the square root of its argument. When we want to find the square root of a number we simply pass it to this method as its sole argument. The value it returns is the root of the number we send it: Math.sqrt(4) Math.sqrt(9) Math.sqrt(16) Math.sqrt(25)<br /> <br /> // // // //<br /> <br /> returns returns returns returns<br /> <br /> 2 3 4 5<br /> <br /> The ceil(), floor(), and round() Methods JavaScript provides three methods you can use for rounding numbers. The ceil() method is for rounding up, the floor() is for rounding down, and the round() method is for rounding off. The ceil() method is the means of rounding up numbers to the nearest integer. In order to use it, you need to address it through the Math object and send it the number you want to round up as a parameter. Here are some examples:<br /> <br /> 7<br /> <br /> 272<br /> <br /> Day 7<br /> <br /> Math.ceil( 1.0001) Math.ceil( 1.9999) Math.ceil(-1.0001) Math.ceil(-1.9999)<br /> <br /> // // // //<br /> <br /> returns 2 returns 2 returns -1 returns -1<br /> <br /> If you want to round down a number, then you need to use the floor() method. It is used in exactly the same way as the ceil() method—a single number is provided as the argument of the method. Here are some examples: Math.floor( 1.0001) Math.floor( 1.9999) Math.floor(-1.0001) Math.floor(-1.9999)<br /> <br /> // // // //<br /> <br /> returns 1 returns 1 returns -2 returns -2<br /> <br /> Of course sometimes you will need to round up numbers if their fractional part is greater than a half, but if it is less, then you will want to round it down. This is where the round() method comes in. When you round off numbers in this way a choice needs to be made about which side a number with a fractional part of exactly one half will be rounded. In the case of JavaScript, the choice was made that it should be rounded up. To demonstrate the use of the round() method, here are some more examples: Math.round(1.0001) Math.round(1.4449) Math.round(1.5000) Math.round(1.5001) Math.round(1.9999)<br /> <br /> // // // // //<br /> <br /> returns returns returns returns returns<br /> <br /> 1 1 2 2 2<br /> <br /> Note that when you pass a data container such as a variable as the parameter to one of the three rounding methods the data container’s original value is not changed. Therefore if you want to use the rounded number at some point later in a script, you will need to store the result.<br /> <br /> Finding Maximum and Minimum Values Using the max() and min() Methods To find the maximum or minimum number in a series of numbers, we could of course write a script—perhaps as a function to make reuse easier. This would likely involve storing the numbers in arrays and string comparisons. As you might imagine, it could all get a bit messy. Thankfully we don’t need to script that ourselves because the Math object provides a couple of methods. The max() and min() methods are fairly self-explanatory in what they do. Listing 7.12 shows the max() and min() methods in use.<br /> <br /> Numbers and Math<br /> <br /> INPUT<br /> <br /> 273<br /> <br /> LISTING 7.12<br /> <br /> Finding the Smallest and Largest Numbers Among Three Values (FindMaxAndMin.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Finding the maximum and minimum number</title> <script type=”text/javascript” language=”javascript”> <!-- // function SetFocus(){ document.SimpleForm.FirstInput.focus(); } // end function SetFocus() function FindMaxAndMin(){ var num1 = document.SimpleForm.FirstInput.value; var num2 = document.SimpleForm.SecondInput.value; var num3 = document.SimpleForm.ThirdInput.value; if (isNaN(num1) || isNaN(num2) || isNaN(num3)){ alert(“You made an invalid entry. Please start again.”); document.SimpleForm.reset(); SetFocus(); } // end if else { // all entries are valid numbers var MaxNum = Math.max(num1,num2,num3); var MinNum = Math.min(num1,num2,num3); var alertString = “You entered “ + num1 + “, “ + num2 + “ and “ +num3; alertString += “\nThe largest number is “ + MaxNum; alertString += “\nThe smallest number is “ + MinNum; alert(alertString); document.SimpleForm.reset(); SetFocus(); } // end else } // end function FindMaxAndMin() // --> </script> </head> <body onload=”SetFocus()”> <form name=”SimpleForm”> <table> <tr> <td width=”25%” align=”right”>Enter first number:</td> <td><input name=”FirstInput” type=”text”></td> </tr> <tr> <td width=”25%” align=”right”>Enter second number:</td> <td><input name=”SecondInput” type=”text”></td> </tr><br /> <br /> 7<br /> <br /> 274<br /> <br /> Day 7<br /> <br /> LISTING 7.12<br /> <br /> continued<br /> <br /> <tr> <td width=”25%” align=”right”>Enter third number:</td> <td><input name=”ThirdInput” type=”text”></td> </tr> <tr> <td width=”25%” align=”right”> </td> <td><button type=”Button” onclick=”FindMaxAndMin()”> Click to calculate</button></td> </tr> </table> </form> </body> </html><br /> <br /> ANALYSIS<br /> <br /> In the example in Listing 7.12, we used an XHTML form to collect three numbers which are to be processed to find the maximum and minimum of the three<br /> <br /> values. The page opens with the focus set on the first <input> element in the form, by means of an onload attribute on the <body> element which calls the SetFocus() function. The user enters three values, one in each of the <input> elements. Each of the values entered is assigned to three variables—num1, num2, and num3. If even one of the three variables is not a number, which we test by means of the isNaN() function, then an error message is output, the form is reset, and the focus is again placed on the first <input> element in the form. If three valid numbers are entered then a string, held in the alertString variable, is displayed using an alert box. The three numbers entered are listed in the order in which they were entered and the maximum and minimum values of the three numbers are displayed. Once the user has acknowledged the alert box then the user can enter three more numbers.<br /> <br /> Note<br /> <br /> The text you want to add to the <button> element must be placed between the <button> start tag and a </button> end tag. If you try to use a <button/> tag with attributes, then you will have only a tiny button with no text displayed on screen.<br /> <br /> Numbers and Math<br /> <br /> 275<br /> <br /> Creating Random Numbers Using the random() Method Possibly the most fun method we have belonging to any JavaScript object is the Math object’s random() method, although it can be put to serious practical uses too. The random() method generates a random number between 0 and 1. So, if we wanted to create random numbers greater than 1, we would generate a random number between 0 and 1 and multiply by an appropriate number to achieve the correct range of random numbers. Listing 7.13 provides a page where the user can choose a maximum number and one or more random numbers, up to the chosen maximum, will be generated.<br /> <br /> INPUT<br /> <br /> LISTING 7.13 Generating Random Numbers up to a User Selected Maximum (RandomNumbers.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Generating random numbers</title> <script type=”text/javascript” language=”javascript”> <!-- // var MaxNum = 0; var ToDisplay; function SetFocus(){ document.SimpleForm.FirstInput.focus(); } // end function SetFocus() function SubmitMax(){ var num1 = document.SimpleForm.FirstInput.value; if (isNaN(num1)){ alert(“You made an invalid entry. Please start again.”); document.SimpleForm.reset(); SetFocus(); } // end if else { // entry is a valid number MaxNum = document.SimpleForm.FirstInput.value; } // end else } // end function SubmitMax() function GenerateRandom(){ if (MaxNum!==0){ ToDisplay = (MaxNum * Math.random()); ToDisplay = formatNumber(ToDisplay, 0); alert(“Random number up to “ + MaxNum + “ is “ + ToDisplay); document.SimpleForm.reset(); SetFocus();<br /> <br /> 7<br /> <br /> 276<br /> <br /> Day 7<br /> <br /> LISTING 7.13<br /> <br /> continued<br /> <br /> } // end if } // end function SubmitMax() function ClearAndSet(){ document.SimpleForm.reset(); SetFocus(); MaxNum = 0; } function formatNumber(theNum, numDecPlaces) { var num = new String(); num = “” + theNum; var pos = 0; count = 0; while (num.substring(pos-1,pos)!== “.”) { pos += 1 ; count += 1; } while (pos < (count+numDecPlaces)){ pos +=1; } // end while return num.substring(0,pos); } // --> </script> </head> <body onload=”SetFocus()”> <h3>Generate random numbers up to a chosen maximum.</h3> <form name=”SimpleForm”> <table> <tr> <td width=”30%” align=”right”>Enter maximum number:</td> <td><input name=”FirstInput” type=”text”></td> </tr> <tr> <td width=”25%” align=”right”> </td> <td><button type=”Button” onclick=”SubmitMax()”> Submit max. number</button></td> </tr> <tr> <td width=”25%” align=”right”> </td> <td> </td> </tr> <tr> <td width=”25%” align=”right”><button type=”Button” onclick=”ClearAndSet()”>Clear and Start again</button></td> <td><button type=”Button” onclick=”GenerateRandom()”><br /> <br /> Numbers and Math<br /> <br /> LISTING 7.13<br /> <br /> 277<br /> <br /> continued<br /> <br /> Click to calculate random number</button></td> </tr> </table> </form> </body> </html><br /> <br /> ANALYSIS<br /> <br /> The listing is longer than many of the others that you have seen, so let’s take it one step at a time.<br /> <br /> The onload attribute on the <body> element calls the SetFocus() function to set focus on the <input> element named “FirstInput”. In addition, the variable MaxNum is set to zero. When the user enters a number and then clicks on the button labeled “Submit max. number”, the SubmitMax() function is called and the chosen maximum number is stored in the variable MaxNum. Clicking on the button labeled “Click to calculate random number” calls the GenerateRandom() function which, using an if statement to control what happens, generates and displays a random number only if the MaxNum variable is not equal to zero. During the process of displaying the generated random number the GenerateRandom() function calls the formatNumber() function that you saw in Listing 7.1. On this occasion we set the number of decimal places to zero so that an integer number is displayed. If you want to generate random numbers which allow numbers beyond the decimal point, then adjust the second argument of the call to the formatNumber() function. The user can generate multiple random numbers up to the chosen maximum, until such time as a new maximum number is passed to the SubmitMax() function or the form is cleared by clicking the button labeled “Clear and Start again”. The button labeled “Clear and Start again” resets the form and returns focus to the element. In addition, the MaxNum variable is set to zero. Thus, if the button labeled “Click to calculate number” is clicked after the form has been cleared, then it does nothing until a new maximum number has been entered by the user, and the button labeled “Submit max. number” has been clicked.<br /> <br /> <input><br /> <br /> The exp() and pow() Methods The Math object has two methods for raising numbers to a power. The first is specific and is provided as a shortcut to a common mathematical process. This method is the exp() method. It returns Euler’s constant raised to the power of the numeric parameter it is given. For example, as shown here:<br /> <br /> 7<br /> <br /> 278<br /> <br /> Day 7<br /> <br /> // Euler’s constant, e = Math.exp(1) // returns Math.exp(2) // returns Math.exp(3) // returns<br /> <br /> 2.718281828459045 2.718281828459045 7.38905609893065 20.085536923187668<br /> <br /> In the first example, e is raised to the power of one, which of course simply is the value of e itself. In the second example, e is raised to the power of 2 (e squared), and in the third and final example, e is raised to the power of 3 (e cubed). This may be a handy shortcut for some advanced math, but we also have a more general method for raising other numbers to powers that we specify. To do this we use the pow() method. With the pow() method, you must specify two parameters. The first is the number that you want to raise, and the second is the number that you want to raise it by. Here are some examples just to make sure that is clear: Math.pow(2,2) Math.pow(2,3) Math.pow(2,4) Math.pow(2,5)<br /> <br /> // // // //<br /> <br /> returns 4 returns 8 returns 16 returns 32<br /> <br /> The first line is 2 raised to the power of 2 (2 squared), the second line is 2 raised to the power of 3 (2 cubed), and the third and fourth lines are 2 raised to the power of 4 and 5 respectively. These numbers can of course be any values you want to use. This includes raising numbers to negative powers (which is the same as 1 over the number raised to the same positive power). The following shows you some examples: Math.pow(2,-2) Math.pow(2,-3) Math.pow(2,-4) Math.pow(2,-5)<br /> <br /> // // // //<br /> <br /> returns returns returns returns<br /> <br /> 0.25 0.125 0.0625 0.03125<br /> <br /> (1/Math.pow(2,2)) (1/Math.pow(2,3)) (1/Math.pow(2,4)) (1/Math.pow(2,4))<br /> <br /> Creating Your Own Math Functions Although the Math object has many methods, if you use math extensively in your scripts you will probably run across situations where you need a function that isn’t present as a method of the Math object. If this happens we would encourage you to think about how you could use the methods that the Math object does have to create your own functions. We have included three functions that you may find useful. If you don’t foresee yourself using a lot of math on your Web site, feel free to skip these examples and come back to them at a later time, if you find you need them after all.<br /> <br /> Finding the Factorial of a Number The factorial of a number can be calculated using the ability of functions to recurse (call themselves repeatedly), as shown here:<br /> <br /> Numbers and Math<br /> <br /> 279<br /> <br /> function factorialOf(theNum) { if (theNum > 0) return theNum * factorialOf(theNum-1); else return 1 }<br /> <br /> Let’s follow the flow through the factorialOf() function. If the number that is the function’s argument is 0 then the number 1 is returned. If the theNum argument is greater than zero, then the fourth line of the factorialOf() function calls itself but with an argument one less than the theNum argument. This process is called recursion. Let’s look at what happens if the argument passed to the factorialOf() function is 3. The number to be returned is 3 multiplied by factorialOf(2), which returns 2 and calls the factorialOf(1), which returns 1 and calls factorialOf(0), which returns 1. Thus factorialOf(0) returns 1, which is multiplied by the value returned by factorialOf(1), which is 1, which in turn is multiplied by the value returned by factorialOf(2), which in turn is multiplied by the number within factorialOf(3). Thus we have the following: 1 * 1 * 2 * 3<br /> <br /> So the expression factorialOf(3) returns the value of 6. When using recursion you need to be sure that there is some situation where recursive calls stop being made. In this example, when the argument reaches zero the value returned is 1 and no further recursive function calls are made.<br /> <br /> Finding the nth Root Another function that may come in useful is being able to find roots of a number other than the square root. Before we try to write a function to do this, let’s consider how we might do it with math notation. To demonstrate, consider the following expression that contains three numbers x, y, and z: x = y ^ z<br /> <br /> In this example, let’s assume that we know the values of x and z, and we need to find the value of y. If we were using a calculator, we would find the zth root of x. But how would we do it if our calculator only had a square root button (as JavaScript does)? Well we could cancel off the power of z from the righthand side by raising it to one over z. However, if we do that then we also have to raise the lefthand side to one over z, as shown here: x ^ (1/z) = y ^ z ^ (1/z)<br /> <br /> 7<br /> <br /> 280<br /> <br /> Day 7<br /> <br /> On the righthand side the z ^ (1/z) reduces to 1 and we are simply left with y. We now have a formula for finding y, which if we swap the sides is as follows: y = x ^ (1/z)<br /> <br /> Therefore the zth root of x is the same as raising x to the power of one over z. There is a method belonging to the Math object which can do this, as shown here: function nthRootOf(theNum, theRoot) { return Math.pow(theNum, 1/theRoot) }<br /> <br /> Later in the book you will see how you can add this function to the Math object as one of its methods.<br /> <br /> Log to Base N The proof for the following function will not be included, but we have included the function as you may find it useful at some point. The Math object has a method that finds the natural log (log to base e) of the number you pass to it as a parameter. Unfortunately it does not include a method for the other common log function which is log to base 10. This is quite easily solved. To find the log of a number to any base, you simply need one log function. It doesn’t matter which base it is for. We already have the method for the natural log, which will do nicely. As long as we use the same log function, if we divide the log of one number by the log of another number, the result is the same as if we had taken the log of the first number to the base of the second. Therefore if we were to use JavaScript to find the log to base 10 of x, we would simply write the following: Math.log(x) / Math.log(10)<br /> <br /> We could of course incorporate this into a function, given a suitable name such as “log10()”. However we can make it even more general and create a function that will find the log of any number to any base. To do this, you could write something like the following: function logN(theBase, theNum) { return Math.log(theNum) / Math.log(theBase); }<br /> <br /> There are many, many more things you can do with JavaScript from a mathematical point of view. We hope you are starting to feel comfortable with the ways these objects and their methods and properties can be used in Web pages where calculations are needed.<br /> <br /> Numbers and Math<br /> <br /> 281<br /> <br /> Summary In this chapter we have gone beyond simple arithmetic operators and introduced you to the use of the JavaScript Number and Math objects for numerical calculations. We also looked at the properties and methods of the Number object and created some examples that used the Number object. In addition you have been introduced to the properties and methods of the Math object which can be very useful in a variety of situations where mathematical calculations are needed.<br /> <br /> Workshop In this workshop we will review some of the new material that you have learned about the Number and Math objects in this chapter.<br /> <br /> Q&A Q. True or False. The Number object has a NaN property which you can use to test whether or not a value is a number. A. False. You need to use the isNaN() function to test whether or not a value is a number. Q. What property of the Math object would you use when calculating the area of a circle? A. The Math.PI property is used in the calculation of the area of a circle. The area of a circle is PI times the radius squared.<br /> <br /> Quiz 1. The Number object has a value property. True or False? 2. Which methods of the Math object allow you to find the largest and smallest of a set of numbers? 3. Which property of the Math object is used in the calculation of the area of a circle? 4. How can you create random numbers using the Math object?<br /> <br /> Quiz Answers 1. False. To access the numeric value of a Number object, you use the valueOf() method. 2. To find the largest and smallest of a set of numbers, you would use Math.max() and Math.min().<br /> <br /> 7<br /> <br /> 282<br /> <br /> Day 7<br /> <br /> 3. The PI property of the Math object is used to calculate the area of a circle. The area of a circle is the radius squared multiplied by Math.PI. 4. The Math object has a random() method which finds random numbers between 0 and 1. To create larger random numbers you would need to multiply the number generated by the random() method by the maximum limit within that which you want to generate a random number.<br /> <br /> Exercises 1. Look at Listing 7.1 and adapt it so that instead of using prompt boxes, the numbers are entered using an XHTML form. 2. Create a Web page that uses the JavaScript function that you were shown to raise a number to its nth power.<br /> <br /> WEEK 2 Let’s Take It Further 8 The Browser Issue 9 Date and Time Manipulation 10 Events and Event Handling 11 Dynamic HTML 12 Windows and Frames 13 Regular Expressions Make It Easier 14 Advanced Array Management<br /> <br /> WEEK 2<br /> <br /> DAY<br /> <br /> 8<br /> <br /> The Browser Issue JavaScript runs on almost all of the current versions of Web browsers available today, including specifically Microsoft Internet Explorer and Netscape Navigator. Unfortunately, each browser that allows for JavaScript scripting does not support the same JavaScript features. In fact, the versions of JavaScript supported by the different browsers varies from browser to browser. This chapter looks at the differences in the various versions of JavaScript supported by browsers—specifically the differences between Internet Explorer and Netscape Navigator. We also will discuss how to design your JavaScript code for cross-browser compatibility. Finally, we will look at the W3C DOM developed by the World Wide Web Consortium as a common document object model and how you can use it for accessing specific elements of the Web page. This chapter will teach you: • The differences between JavaScript versions supported by different browsers • Cross-browser compatible scripting • About the W3C DOM<br /> <br /> 286<br /> <br /> Day 8<br /> <br /> Different Browsers, Different JavaScript As we have previously discussed, JavaScript differs from most other programming languages in that it is a scripting language, not a compiled language. Languages, such as C++ and Java, are used to create program code that is compiled allowing it to run on virtually any machine. Typically, the operating system where the code will run (such as Microsoft Windows or the Macintosh OS) is the developer’s only consideration when distributing the compiled code. A scripting language, on the other hand, relies on the browser to interpret the scripted code on the Web page. In other words, each line of JavaScript code is interpreted at runtime by the browser that accesses the page containing the code. Because of this factor, it is obvious that JavaScript code can only run on a browser that supports JavaScript. The most popular browsers today, Netscape Navigator and Internet Explorer both provide strong support for JavaScript, but unfortunately the JavaScript features supported by these browsers varies.<br /> <br /> JavaScript History JavaScript originally got its start as a scripting language for Netscape Navigator. Netscape originally recognized the need for a scripting language to do the following: allow Web server administrators to manage the Web server and connect its pages to other services, provide Web page authors the ability to create scripts to run on a Web page and perform tasks such as verifying a value typed into a field, and create an interface for communicating with Java applets placed on an HTML page. This scripting language was originally called “LiveScript.” Before the release of Navigator 2, Netscape and Sun formed an agreement to call the new scripting language JavaScript. JavaScript was then introduced by Netscape as part of Netscape Navigator 2. When Navigator 3 was released, it included an updated version of JavaScript, JavaScript 1.1. At the same time Netscape was releasing Navigator 3, Microsoft released Internet Explorer 3 with its own version of JavaScript named JScript. Because the Java name was trademarked by Sun, Microsoft chose to call it JScript to avoid the need to license the name from Sun. Although JScript was intended to parallel the capabilities of the JavaScript 1.1 available with Navigator 3, its functionality more closely resembled that which was available in the original version of JavaScript released with Navigator 2. Finally, with the release of Navigator 4 and Internet Explorer 4, JavaScript and JScript essentially mirrored core functionalities. Although Microsoft continued to call its version JScript, Internet Explorer script tags recognized both JScript and JavaScript, allowing<br /> <br /> The Browser Issue<br /> <br /> for both types of scripting. Of course, that was not true of Netscape Navigator, which only recognizes JavaScript script tags. In an effort to create a more standardized core language, the European Computer Manufacturers Association (ECMA) met with Microsoft, Netscape, and other organizations with JavaScript interests. Formal language specifications were published as standards for the language. Because of the licensing issues with the name JavaScript, the new language was named ECMAScript. Currently the ECMA maintains the standards for the core language. The core language typically maintains the most compatible features consistent between Netscape Navigator and Microsoft Internet Explorer. Therefore, you can generally feel confident that everything within the ECMA standard will work on the current browser versions for both Navigator and Internet Explorer, but may not be backward compatible to previous browser versions, or any of the many other browsers available. Unfortunately the ECMA standard does not include specifications for all the fun and exciting features you find on various Web sites that you visit today. Most of these extended features are built in to the browsers by Microsoft and Netscape. In order to take advantage of these features, you potentially limit the capability of your audience to view your site in all its glory. One good example of this would be when Navigator 3 provided additional capability with the Browser object that allowed Web developers to create rollover button effects like those which you see on most current sites today. Microsoft was slower to add this type of feature. So Internet Explorer users did not see the rollover effects when visiting sites that had incorporated that feature. Of course this functionality was later added to Internet Explorer. As you can probably deduce, the total functionality available is based not only on the version of JavaScript supported by the browser but also the browser version and its browser object model.<br /> <br /> Note<br /> <br /> You can refer to Appendix E, “A Short History of JavaScript,” for a more detailed history.<br /> <br /> ECMA Standards As mentioned earlier, the ECMA maintains the standards for the core JavaScript language, called ECMAScript because of licensing issues with Sun. Both Netscape Navigator and Microsoft Internet Explorer maintain that they are ECMA-compliant, meaning that they include all of the core language features as outlined by the ECMA standard. The ECMA standards body maintains a document called ECMA-262, which contains all core ECMAScript language standards. You can find the latest version of this document at http://www.emca.ch.<br /> <br /> 287<br /> <br /> 8<br /> <br /> 288<br /> <br /> Day 8<br /> <br /> It is important to recognize that the ECMA provides only core language standards. It is important to be familiar with some of these standards so that you can determine more quickly if the JavaScript functionality you want to add will be supported by all current browsers. For example, EMCAScript Edition 3 specifies the following core components: • Types—There are nine different types that can be used in expressions: Undefined, Null, Boolean, String, Number, Object, Reference, List, and Completion. • Objects—There are 11 different object types that are part of the core language standard: Global, Object, Function, Array, String, Boolean, Number, Math, Date, RegExp, and Error objects. As you work with different browsers, you will find that each browser has its own set of objects for accessing the components of the browser window. • Keywords—The following words are used as keywords: break, case, catch, continue, default, delete, do, else, finally, for, function, if, in, instanceof, new, return, switch, this, throw, try, typeof, var, void, while, with. • Reserved Words—These words have been reserved for future use within JavaScript and therefore should be avoided within your scripts: abstract, boolean, byte, char, class, const, debugger, double, enum, export, extends, final, float, goto, implements, import, int, interface, long, native, package, private, protected, public, short, static, super, synchronized, throws, transient, volatile.<br /> <br /> Cross-Browser Compatible Scripting Because your Web site has the potential to be accessed by multiple browsers, your best bet is to create cross-browser compatible code. What does this mean? Essentially, if your code is cross-browser compatible it should execute reliably on all browsers. On the surface this sounds like a fairly straightforward statement. The problem comes in deciding what you want to use for your standard, for example, browser version, language, or platform. As previously mentioned, the ECMA standard creates a standard for core JavaScript code that will execute reliably on the latest versions of Netscape Navigator and Microsoft Internet Explorer. Since these browsers are the most widely used, it is feasible to assume that the majority of the visitors that will be visiting will be using one of these browsers. However, it also is highly probable that they may not be running the latest version of either browser. Although it is assumed that visitors will have either Navigator or Internet Explorer, this may not necessarily be the case. Especially with the surge of new mobile devices, it is highly likely that many visitors will have browser interfaces that do not support scripting.<br /> <br /> The Browser Issue<br /> <br /> Because this book focuses on JavaScript 1.5, most of the examples in the book are designed to work on browsers that support that version. If you want to code for earlier browsers you can refer to Appendix A, “New Features in JavaScript 1.5,” for the list of new features that were added to JavaScript 1.5. Table 8.1 identifies each of the JavaScript versions and the corresponding browsers that supported them. TABLE 8.1<br /> <br /> JavaScript Version Support<br /> <br /> JavaScript Version<br /> <br /> Browser Support<br /> <br /> JavaScript 1.1<br /> <br /> Netscape 3.01<br /> <br /> JavaScript 1.2<br /> <br /> Netscape 4.05, Internet Explorer 4.01<br /> <br /> JavaScript 1.3<br /> <br /> Netscape 4.61, Internet Explorer 5.0<br /> <br /> JavaScript 1.4<br /> <br /> Mozilla 5.0, Alpha Pre-Release<br /> <br /> JavaScript 1.5<br /> <br /> Netscape 6, Internet Explorer 5.5<br /> <br /> As you can see from the table, JavaScript 1.5 features were supported beginning with Netscape Navigator 6 and Internet Explorer 5.5. Let’s look at ways to identify browsers that don’t support this version of JavaScript, and how to design your code to avoid these problems.<br /> <br /> Browsers That Don’t Support JavaScript Although users can download the latest browser for free from the appropriate site, the chance of being visited by browsers that don’t support JavaScript would appear to be very limited. After all, JavaScript has been supported by Microsoft and Netscape since around 1996. But, as Internet access becomes available on more different devices, such as palm-sized devices and cell phones, it is quite common for sites to be accessed by browsers that do not support JavaScript. These types of browsers run on devices with limited resources, so more demanding features, such as JavaScript support, are not available on most of these devices. Because of the growing potential of your site being accessed by a device that does not support JavaScript, you will want to design your site so that it avoids any of these potential errors. Most older browsers ignore HTML tags that they do not recognize, therefore, when they encounter a <script> tag they typically ignore that tag. But because JavaScript code is not placed within brackets <> those lines of code will be placed on the Web page as text. Because of this situation, we need to use the HTML comment markers around the JavaScript code. By doing this, the non-JavaScript supporting browsers will ignore your<br /> <br /> 289<br /> <br /> 8<br /> <br /> 290<br /> <br /> Day 8<br /> <br /> JavaScript code, but the browsers that support JavaScript will ignore the comment markers and execute the JavaScript code. You place the comment markers in your JavaScript code as follows: <script language=”JavaScript” type = “text/javascript”> <!-window.alert(“JavaScript code goes here.”); //..> </script><br /> <br /> Notice the <!-- tag marks at the beginning of the JavaScript code and the //..> tag marks the end of the script. The browsers that do not support scripting will ignore all code between those tags. Of course, they will also ignore the HTML </script> tag that signals the end of the JavaScript code.<br /> <br /> Caution<br /> <br /> Be aware there are some older America Online browsers that do not ignore the code placed within the HTML comment lines. If a user visits your page using one of these old browsers, your JavaScript code will appear on the Web page. Fortunately, these browsers are very outdated and rarely used at this time.<br /> <br /> Keep in mind, the HTML comment markers do not hide your JavaScript code. They only tell the browser to ignore the code during processing. As you are probably well aware, most browsers allow for the source code of a page being visited to be viewed by the visitor. This will include any JavaScript code that you have embedded within your page.<br /> <br /> Browsers with JavaScript Support Turned Off Even though a browser supports JavaScript, there is still the possibility that the JavaScript support may be disabled for that particular browser. Nearly all browsers that support JavaScript provide the option of disabling JavaScript. Although browsers default to allowing JavaScript, many IS departments choose to disable Java and JavaScript support to eliminate any potential security violations. Even if the browser is set to support JavaScript, the corporate Firewall may attempt to filter out JavaScript lines from the incoming HTML code. It is quite simple to disable the support of JavaScript from a browser as shown in Figure 8.1, which illustrates the Netscape Navigator Preferences window.<br /> <br /> The Browser Issue<br /> <br /> 291<br /> <br /> FIGURE 8.1<br /> <br /> 8<br /> <br /> Netscape allows each user to specify whether to allow JavaScript code to execute.<br /> <br /> To allow for the potential that the scripting support for the browser has been disabled, each of the browsers that support JavaScript provides an additional set of HTML tags— <noscript> and </noscript>. Each browser will execute the code within these tags if the JavaScript support is disabled for the browser. Listing 8.1 illustrates code you can use to notify the user that the JavaScript support has been disabled for his browser.<br /> <br /> INPUT<br /> <br /> LISTING 8.1<br /> <br /> Catching Browsers with JavaScript Disabled (JSDisabled.htm)<br /> <br /> <head> <script language=”JavaScript” type = “text/javascript”> <!-window.alert(“JavaScript is enabled on this browser.”); //..> </script> <title>Check to See if Scripting is Enabled</title> <noscript> <b>Your browser has JavaScript disabled.</b> </noscript> </head> <body> </body> </html><br /> <br /> As you can see in Figure 8.2, if the browser does not have JavaScript support enabled, the code within the <noscript> </noscript> tags will execute. Remember, you cannot place any script within those tags because the browser will not know how to execute it. Therefore, you can only use HTML tags within the tags.<br /> <br /> 292<br /> <br /> Day 8<br /> <br /> OUTPUT FIGURE 8.2 When your page displays on a browser with JavaScript disabled, the HTML code within the <noscript> tags executes.<br /> <br /> Keep in mind that you will want to add the <noscript> tags after each location where you have <script> tags. This instructs the browser as to the code to run if the JavaScript support is disabled.<br /> <br /> Determining Browser Information Another way to create cross-browser code is to determine the browser type and version for the user that is visiting you. You can get browser-specific information from the Navigator object. The Navigator object was originally added by Netscape to Navigator 2.0, but other browsers soon followed suit. It has become a standard for all browsers that support JavaScript, making it another good method to use when determining whether your code will run on a user’s browser. Although the Navigator object does appear on browsers that support JavaScript, it is not a JavaScript object. Also, it is not part of the DOM (Document Object Model) that we will be discussing later in this chapter. In fact, the Navigator object sits alone with the sole purpose of gathering browser-specific information that can be accessed with its properties. You can access a property of the object by typing the following: navigator.propertyname<br /> <br /> There are several different properties available for the Navigator object, as shown in Table 8.2. Although this object was originally implemented by other browsers to be consistent with Netscape, there are properties supported only by Internet Explorer, and others that only Netscape supports. Therefore, to avoid potential issues, the table identifies the browsers that support each property.<br /> <br /> The Browser Issue<br /> <br /> TABLE 8.2<br /> <br /> 293<br /> <br /> Navigator Properties<br /> <br /> Property<br /> <br /> Browser<br /> <br /> Description<br /> <br /> appCodeName<br /> <br /> both<br /> <br /> Returns the internal code name of the browser. Both Navigator and Internet Explorer return a value of Mozilla.<br /> <br /> appMinorVersion<br /> <br /> IE 4.0+<br /> <br /> Specifies the minor version number of the browser. Minor version number changes occur due to patches or service-pack updates.<br /> <br /> appName<br /> <br /> both<br /> <br /> Returns the official name of the browser.<br /> <br /> appVersion<br /> <br /> both<br /> <br /> Returns the version number of the browser.<br /> <br /> browserLanguage<br /> <br /> IE 4.0+<br /> <br /> Identifies the language supported by the browser.<br /> <br /> cookieEnabled<br /> <br /> both<br /> <br /> Returns a Boolean value of True if the browser is set to allow cookies, or False if cookies are not allowed.<br /> <br /> cpuClass<br /> <br /> IE 4.0+<br /> <br /> Specifies the type of CPU being used by the computer where the browser is running.<br /> <br /> language<br /> <br /> Netscape 4.0+<br /> <br /> Identifies the language supported by the browser.<br /> <br /> online<br /> <br /> IE 4.0+<br /> <br /> Returns a value of True if the browser is in online mode. Otherwise, a value of False is returned.<br /> <br /> platform<br /> <br /> both<br /> <br /> Specifies the platform (operating system) on which the browser is running.<br /> <br /> securityPolicy<br /> <br /> Netscape 4.7+<br /> <br /> Specifies the encryption policy specified on the computer.<br /> <br /> systemLanguage<br /> <br /> I.E. 4.0+<br /> <br /> Identifies the default language supported by the user’s operating system.<br /> <br /> userAgent<br /> <br /> both<br /> <br /> Returns a string containing the following property values: appCodeName, appName, appVersion, language, and platform.<br /> <br /> userLanguage<br /> <br /> I.E. 4.0+<br /> <br /> Names the language the user has specified on their system.<br /> <br /> vendor<br /> <br /> Netscape 6.0+<br /> <br /> Returns the name of the company that makes the browser.<br /> <br /> As you can see there are several different properties supported by the Navigator object, depending on which browser you are using. You will probably find additional properties of the Navigator object available only on specific browsers. Typically when using the Navigator object to determine browser-specific information you are going to want to select properties supported by all browsers, such as appName and appVersion.<br /> <br /> 8<br /> <br /> 294<br /> <br /> Day 8<br /> <br /> Probably the easiest method for determining which browser a user has is to use the appName property as illustrated in Listing 8.2.<br /> <br /> INPUT<br /> <br /> LISTING 8.2<br /> <br /> Checking Browser Name (NameBrowser.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-if (navigator.appName == “Microsoft Internet Explorer”) { alert(“Your browser is Microsoft Internet Explorer.”); } else if (navigator.appName == “Netscape”) { alert(“Your browser is Netscape Navigator.”); } else { alert(“You are not using Netscape Navigator or Internet Explorer.”); } //..> </script> <title>Find Browser Name</title> </head> <body> </body> </html><br /> <br /> The code in Listing 8.2 displays an alert window with a message indicating the browser name, as shown in Figure 8.3. You will notice that I added an else statement to accommodate any other browsers that were not Netscape Navigator or Internet Explorer.<br /> <br /> OUTPUT FIGURE 8.3 The appName property returns the name of the browser that is viewing your Web page.<br /> <br /> The Browser Issue<br /> <br /> The final else statement may initially appear to handle all issues, but it doesn’t. Some browsers, specifically Opera, have the ability to customize your browser identification. By changing this setting in the preferences, an Opera browser can appear to be Netscape Navigator or Internet Explorer. Although this setting was added to the browser to ensure access to sites that are coded for a specific browser type, it defeats your purpose for checking.<br /> <br /> ANALYSIS<br /> <br /> Fortunately there is another property that you can use to get this same information, the property. Since this property returns more than the browser name, you have to look in the string to determine what value was returned for the appName value property.<br /> <br /> userAgent<br /> <br /> When you use the userAgent property it returns a string that contains information about the user’s browser. The browser and operating system that are being used to access your site determines which property values are actually returned by the userAgent property. Nearly always the following properties are returned in the string: appCodeName, appVersion, appName, and platform. For example, Figure 8.4 illustrates the string returned by Internet Explorer 6.0 running on Windows XP. FIGURE 8.4 The actual string returned by the userAgent property varies slightly based upon the user’s browser and operating system.<br /> <br /> As you can see in Figure 8.4, the string returned by the userAgent property is fairly lengthy. To determine if a specific browser name is in the string, you will need to search the string. One good method to use is the indexOf() string method, which will determine if one string contains another string.<br /> <br /> Note<br /> <br /> The indexOf() method allows you to compare the two strings, and it returns an integer value indicating the location where the second string is located within the first string. If the string is not found, the method returns a value of -1.<br /> <br /> 295<br /> <br /> 8<br /> <br /> 296<br /> <br /> Day 8<br /> <br /> By combining the indexOf() method with if statements, you can determine which browser is visiting your site, as shown in Listing 8.3.<br /> <br /> INPUT<br /> <br /> LISTING 8.3<br /> <br /> Using the userAgent Property (UserAgent.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-var bname = “browser”; if (navigator.userAgent.indexOf(“MSIE”) != -1) { bname = “Internet Explorer”; } else if (navigator.userAgent.indexOf(“Opera”) != -1) { bname = “Opera”; } else if ((navigator.appName.indexOf(“Netscape”) != -1) || parseFloat(navigator.appVersion >= 3.0)) { bname = “Netscape Navigator”; } else { bname = “Unknown Browser”; } document.write(“Your browser is: “ + bname); //..> </script> <title>Display Browser Information</title> </head> <body> </body> </html><br /> <br /> You will notice that we still had to use a different property combination to check for Netscape. When working with the userAgent property, the application name (Netscape) is not part of the userAgent string for Netscape Navigator versions prior to 6.0. Therefore, you cannot just use the userAgent property to check for “Netscape” if you want to detect Netscape versions prior to 6.0. Figure 8.5 shows how the code detects the browser name.<br /> <br /> ANALYSIS<br /> <br /> The Browser Issue<br /> <br /> OUTPUT FIGURE 8.5 The userAgent property provides another method for determining the actual browser type.<br /> <br /> Of course, instead of concentrating on the browser name, you may be more interested in looking at the browser version to determine if your JavaScript code will execute properly. You can check the browser application version using the appVersion property. If you remember from Table 8.1, JavaScript support was added to Netscape Navigator version 3.0 and to Internet Explorer 4.01. Therefore, by making sure the browser version is greater than 3.0 for Netscape and 4.01 for Internet Explorer, you are assured that JavaScript is supported by those browsers. We did this type of check when looking for Netscape in the preceding Listing 8.3 above.<br /> <br /> Determining Browser Support for Specific Objects Another method for creating cross-browser scripting is to use specific object support to determine whether or not your code will execute on the specific browser. So what exactly does that mean? Even though Netscape Navigator and Internet Explorer both have objects for the various elements of the browser window, not all of the objects are supported by both browsers. Also, remember that even objects (such as the Document object), which both browsers support, do not always have the same properties on both browsers. Therefore, by checking for specific object property values you are able to ensure that your browser supports your JavaScript code. For example, both Netscape and Internet Explorer have an Event object that tracks information about each event. Although they both support the same object, they do not support the same properties. In fact, Netscape Navigator supported different properties for the object with versions 4 and 5 than with version 6. Therefore, in order to use this object without errors on the other browsers, you need to create code to check for browser support. One good way to do this is to use the Document object. This is a common method used to distinguish between Internet Explorer, Navigator 6, and Navigator 4. To do so you can use if statements to determine which document object property is supported, as shown in the following code: if (document.all) //Checks for IE 4.0 or later {<br /> <br /> 297<br /> <br /> 8<br /> <br /> 298<br /> <br /> Day 8<br /> <br /> document.form1.text2.value = String.fromCharCode(event.keyCode); } else if (document.getElementById) //checks for Netscape 6 or later { document.form1.text2.value = String.fromCharCode(key_event.which); } else if (document.layers) //Checks for Netscape 4 { document.form1.text2.value = String.fromCharCode(key_event.which); } }<br /> <br /> You will notice that although each one of the assign statements are assigning the key pressed to the object, a different property is being used for each.<br /> <br /> Note<br /> <br /> You can find out more about the Event object in Chapter 10, “Events and Event Handling.”<br /> <br /> The W3C DOM In Chapter 4 we looked at how JavaScript is object-based allowing you to access various elements of the browser by changing the properties of a particular object. The objects or elements of the browser are all part of the Document Object Model (DOM). Initially both Microsoft Internet Explorer and Netscape Navigator developed their own independent DOMs, making object scripting reliant on one specific browser. To help standardize the object models to allow developers to create object-oriented scripting for multiple browsers, the World Wide Web Consortium (W3C) developed a Document object specification commonly referred to as the W3C DOM. The DOM describes the relationship of all objects on the HTML page. The relationship resembles a tree-like structure. Each object within the DOM is referred to as a node. The first element on the tree is the Document object, and the tree structure branches out from there. For example, the <HTML> tag is a node of the document, and the <Title> and <Body> tags are child nodes of the <HTML> tag. Of course you are aware that the Document object does not appear in the actual HTML code listing. The Document object is always loaded with the page as the first element for the page. Again every element of the page is a node, including text which is referred to as a Text node. A node that contains other nodes, including text, is referred to as a parent node. A node contained within another node is a child node. Therefore each line of code you<br /> <br /> The Browser Issue<br /> <br /> 299<br /> <br /> specify creates nodes. For example, the following HTML code creates two different nodes: <p>Paragraph Text</p><br /> <br /> The code creates a p element and a text node containing “Paragraph Text.” Since the text node is inside of our element p node, it is considered to be a child node. To better understand the node concept of the W3C DOM let’s look at the following code: <html> <head> <title>Header Text</title> </head> <body> <p><i>Body Text</i></p> <div> <a href =”link.htm” rel="nofollow">link text</a> <img src = “test.gif”> </div> </body> </html><br /> <br /> If you look at this code closely, you will see that the HTML tags enclose the entire document code. The html node is a child of the Document node. In fact, the HTML node is the only child of the Document node. The Head and Body nodes are child nodes of the html node, as illustrated in Figure 8.6. FIGURE 8.6 The W3C DOM treats each element of the HTML document as a separate node, creating a treelike structure.<br /> <br /> document html<br /> <br /> head<br /> <br /> Body<br /> <br /> title<br /> <br /> p<br /> <br /> Header Text<br /> <br /> i<br /> <br /> Body Text<br /> <br /> div<br /> <br /> a<br /> <br /> img<br /> <br /> Link Text<br /> <br /> Another thing to notice with the representation in Figure 8.6 is that the DOM also tracks sibling relationships. For example, the img and a nodes are siblings because they belong to the same parent node (div).<br /> <br /> 8<br /> <br /> 300<br /> <br /> Day 8<br /> <br /> Okay, I am sure that all sounds good, but you are probably wondering what nodes really have to do with creating Web pages. After all, you have been able to do a lot of coding up to now without dealing with different node issues. But like I mentioned at the beginning of this section, the DOM is designed to enable you to have a more dynamic page by interacting with elements of the browser page. The DOM nodes provide a method for accessing and modifying various objects on the page.<br /> <br /> Accessing Nodes Once you understand the concept of a node as specified by the W3C DOM, we can look at how to access specific nodes or elements of the page. As we mentioned earlier, each node has a child node. For example, html is a child node of the Document node. The child nodes for a node are accessed in an array-like fashion using the childNodes collection. For example, to access the html node from the Document node you would type the following: document.childNodes[0]<br /> <br /> Notice that we use 0 to access the html node. This is because the values within a collection start at 0. As a result, you can expand the reference to access the first child of the html node by typing: document.childNodes[0].childNodes[0]<br /> <br /> This line would access the Head node of the document. There are two additional shortcut properties you can use to access the first and last child nodes. For example, to access the first child node for the Document object you can type: document.firstChild<br /> <br /> Again, this code accesses the html node because that is the only child of the Document object. You can also use the lastChild property to get the last child node for a parent node. In much the same fashion that we stepped down through the tree, you can also back up and determine the parent node for any node. For example, assume we have assigned the Title node value to the titlenode variable as follows: titlenode = document.childNodes[0].childnodes[0]<br /> <br /> Later we can return the parent to the Title node by typing the following: titlenode.parentNode<br /> <br /> The Browser Issue<br /> <br /> It doesn’t take long to realize that references to nodes within a document can get quite long as you step through the different levels. To solve this issue, the DOM provides methods for specification based on the element identification or the HTML tag name.<br /> <br /> Accessing an Element by ID Rather than traversing the nodes to find the element that you want to access, you also can access an element using the Id attribute assigned to the HTML element. To do this you use the getElementById method. For example in Listing 8.4, we illustrate how this method can be used to return the value of a text input field on a form.<br /> <br /> INPUT<br /> <br /> LISTING 8.4 Using the getElementById() method (GetElementByIDMethod.htm)<br /> <br /> <html> <head> <title>New Page 1</title> <script language=”JavaScript” type = “text/javascript”> <!-function Test() { alert(document.getElementById(“testinput”).value); } //..> </script> </head> <body> <form name=”textfield”> <input type=”text” name=”T1” size=”20” id=”testinput”> <input type=”button” value=”Click Here” name=”B1” onclick=”Test()”></p> </form> </body> </html><br /> <br /> ANALYSIS<br /> <br /> Notice that we are referencing the text input field with the code: document.getElementById(“testinput”).value<br /> <br /> This method makes accessing the value of the form field much easier than traversing the tree of document nodes as illustrated earlier.<br /> <br /> 301<br /> <br /> 8<br /> <br /> 302<br /> <br /> Day 8<br /> <br /> Note<br /> <br /> Because the getElementById() is a method that is specific to the W3C DOM it is frequently used to verify a browser’s W3C compliance, as illustrated with the following code: if (document.getElementById) { document.write(“Browser is DOM compliant”); } You can use this type of compliance check to identify Internet Explorer 5 and later or Netscape 6 and later which are both W3C DOM compliant.<br /> <br /> Accessing Elements by Tag Name Another method you can use to quickly access individual elements of a page is to access the elements with the same tag name. For example, you could access all p tags, or all a tags. This can be accomplished using the getElementByTagName method. When you use this method it returns a collection of all elements with the specified tag name. For example, if you have the following code: <p>Paragraph 1</p> <p>Paragraph 2</p> <p>Paragraph 3</p><br /> <br /> You can access the first p tag by typing document.getElementsByTagName(“p”)[0]. Keep in mind, this method is only referencing the actual tag name and not a value specified with the Name attribute for an HTML element.<br /> <br /> Note<br /> <br /> Of course accessing the nodes of your HTML document is only a small portion of what can be accomplished with the W3C DOM. Working with DHTML you can perform more powerful options such as changing the attributes of a tag, adding an attribute to a tag, or even removing tags. You can even change styles of text. We look at these options in more depth in Chapter 11 where we discuss DHTML.<br /> <br /> Summary In this chapter we have looked at a lot of different aspects of JavaScript and the browsers where your code runs. We explored the fact that different browsers support different versions of JavaScript, making it a challenge to determine what JavaScript features to use when developing your Web page.<br /> <br /> The Browser Issue<br /> <br /> We briefly looked at the history of JavaScript and how it has evolved. We discussed that the ECMAScript Standard maintains the standards for the JavaScript supported by all browsers. Although this is the standard, each browser provides additional JavaScript features. Next we looked at how to determine if a browser supported JavaScript, and if the JavaScript code was active. We looked at how the browser information can be determined using the Navigator object. Using the properties associated with the Navigator object you can determine which browser a user has when he connects to your site and execute specific code based upon that browser’s settings. We also looked at how you can also use properties of the Document object to determine which code to run. Finally, we began looking at the W3C DOM and how it provides a standard Document Object Model for each browser. Using the DOM you can access each element of your Web page as a node. This foundation on the DOM sets the stage for the DOM work we will do in Chapter 11 when we work with DHTML.<br /> <br /> Workshop In the workshop today we will use the questions to review what you have learned in this chapter about cross-browser compatibility issues and working with the W3C DOM.<br /> <br /> Q&A Q. How do I handle code for each specific browser? A. As we have discussed, different browsers and browser versions support different JavaScript features. This means that creating code to run for each scenario can become quite complex if it is all maintained on one page. A common method is to call a separate page for each browser. For example, load one page for Netscape and another for Internet Explorer. Q. What do I do about browsers other than Netscape Navigator and Internet Explorer? A. There have been an abundance of different browsers that have surfaced over the years, and more will continue to come out. In this chapter we concentrated on the two most commonly used browsers: Netscape Navigator and Microsoft Internet Explorer. You should make a point to support the most widely used browsers in your script. Another browser that has gained popularity that you may want to support is Opera. This browser also supports JavaScript. You can find more information about this browser at www.opera.com.<br /> <br /> 303<br /> <br /> 8<br /> <br /> 304<br /> <br /> Day 8<br /> <br /> Q. Can I design my Web page to capture more specific information about the user, such as name or e-mail address? A. No, the Navigator object that we discussed in this chapter is only able to provide information about the user’s browser and operating system. The only way to get any more specific information about a user is to use a signed script. We’ll look at the use of signed scripts in Chapter 17, “Privacy and Security.”<br /> <br /> Quiz 1. What is the ECMAScript Standard? 2. How do you ensure your script is ignored on browsers that don’t support JavaScript? 3. What tags do you use for browsers with JavaScript disabled? 4. What object can you use to gather information about the user’s browser?<br /> <br /> Quiz Answers 1. The ECMAScript Standard specifies the JavaScript standards that should be supported by all browsers. Because the ECMAScript Standard is typically behind the times based upon the features of the new browser versions, browsers typically provide additional features that are not part of the ECMAScript Standard. Because these new features are not part of the standard, you need to exercise caution when using them to ensure that they will run properly on all browsers. 2. When placing JavaScript on your Web page you need to place the HTML comment tags <!-- and //..> within the <script> tags to mark the block of code to be treated as comments by non- JavaScript supported browsers. When a browser that does not support JavaScript encounters the <script> tags, it recognizes they are HTML tags it does not support, so they are ignored. The actual code must be placed within the comment tags so that the browser does not write the code to the page as text. 3. Any browser that supports JavaScript includes the <noscript> tags to be used to specify the code that should execute if the JavaScript option is disabled. Remember, you want the code placed within these <noscript> tags to be HTML only. Since the JavaScript has been disabled the only type of code that will execute is HTML. 4. The Navigator object is available with all current browsers. This object maintains information about the current browser. You can use the properties associated with this object to return specific information about the object. For example, the appName property returns the name of the current browser.<br /> <br /> The Browser Issue<br /> <br /> 305<br /> <br /> Exercises 1. Use the Navigator.appVersion property to determine if the browser supports JavaScript. For a list of browser versions that support JavaScript, refer to Table 8.1. Also, include the code to run, if the JavaScript support is not active. 2. Create a W3C-DOM page that only executes on Web pages that are DOM-compliant. Use the nodes to access the value which is typed in the second form field and displayed in an Alert window.<br /> <br /> 8<br /> <br /> WEEK 2<br /> <br /> DAY<br /> <br /> 9<br /> <br /> Date and Time Manipulation As you are aware, dates and times are a key part of each of our lives. In fact, our lives revolve around specific times that indicate the occurance of events. This holds true when dealing with Internet information as well. Typically everything has a date and time correlation—whether it is the date when a particular item is on sale, hours of operation for a business, or even just displaying current date and time information on your Web page. This chapter will look at working with dates and time within JavaScript; and, more specifically, how to work with the Date object. This JavaScript object is responsible for maintaining not only the date information but also the corresponding time information. This chapter will teach you • What the Date object is • How to format dates • How to format times • How to convert date and time formats<br /> <br /> 308<br /> <br /> Day 9<br /> <br /> JavaScript and Dates On the surface the concept of working with dates and times seems very straightforward. After all, we all learned in grade school how to interpret 4:30 p.m. But the difficulty with dates in JavaScript comes from understanding how JavaScript calculates dates. JavaScript stores all dates as numeric values. The date value is actually the number of milliseconds that have passed since 12:00 a.m., January 1, 1970. In other words, all dates and times since that date are stored as a numeric value, where 10 represents 10 milliseconds after midnight on January 1, 1970. Any dates and times prior to that date are represented as negative values, counting backwards from that date and time. Because JavaScript stores all dates and times as numeric values, calculations to compare dates are quite simple because you are basically just comparing two numeric values. For example, if you want to determine the amount of time that has elapsed between two dates, you are actually performing a simple mathematical calculation by subtracting one date from the other date. When JavaScript executes it always pulls the date and time information from the user’s computer. Therefore, the date and time information is only as accurate as the settings on an individual’s computer. Although JavaScript displays times based on the user’s current computer settings, it actually stores and manipulates all dates based on Greenwich Mean Time (GMT). If you are not familiar with GMT, it is the time zone from which all other times zones are calculated. This time zone, which is situated over Greenwich, England, gets its name from its location. So, for example, Central time is six hours behind GMT. Actually JavaScript’s treatment of the dates is handled basically in the same fashion as your computer. Your personal computer’s clock settings are based on GMT. Typically when you install an operating system, such as MS Windows, on your computer, you are requested to specify the time zone where you live. For example, if you are in the Central time zone your machine knows that it has to use the offset from GMT to calculate your local time frame, in this case, six hours behind GMT. If you happen to live in an area that recognizes daylight savings time, your machine also maintains a separate offset that it uses during that time of the year. Therefore, when you request the date and time from your computer, JavaScript is capturing the time from the personal computer’s system clock. The date values are converted from the user’s current date and time. Again, this all assumes that the system clock is correct.<br /> <br /> Date and Time Manipulation<br /> <br /> 309<br /> <br /> Each browser stores all date and time information in a single Date object that you can access from your JavaScript code. In order to access any date or time, you have to access a specific Date object, as covered in the next section.<br /> <br /> The Date Object In order to work with dates and times in JavaScript, you must use the Date object. Although the Date object is one of the JavaScript Core objects, there is an instance of the object created for each browser window (that recognizes JavaScript), or frame. The Date object always represents one specific instance of time, such as Apri1 2, 2001, 4:15 p.m. You can specify a range of time with the Date object. Typically when working in JavaScript, you are going to use the Date object to grab the current date and time from the user’s system settings. Therefore, the date information that is captured is only as accurate as the system clock. Therefore, avoid relying on specific date and time information within your program. For example, if you plan to launch a special greeting on a certain date, if the system clock on the user’s computer is not properly set your greeting may never display. To capture the date and time information, you typically assign the Date object value to a variable. Once you do this, you can make desired modifications to the date information, such as changing the formatting. var currentdate = new Date();<br /> <br /> Each time JavaScript encounters the new Date() statement, it assigns the current date and time information to the corresponding variable. Again, be aware that the date information is captured at the point in your code where JavaScript encounters the statement. If, for example, the date was captured at the beginning of your code, but your code took two minutes to execute, the variable would still contain the time information from when the date and time was captured with the Date object. Don’t get confused with the name of the object. Even though the object is called Date, as we have previously discussed, the object is used to capture both the date and time information. Therefore, even if you only want to know the current date, the object still will contain the time information, to the exact millisecond from when the Date object was created. The way the date information displays on your page is ultimately governed by the default date formatting of the browser. To display the actual system date information, you would create a script similar to the code in Listing 9.1.<br /> <br /> 9<br /> <br /> 310<br /> <br /> Day 9<br /> <br /> INPUT<br /> <br /> LISTING 9.1<br /> <br /> Displaying the Current Date (specifydate.htm)<br /> <br /> <script language=”JavaScript” type = “text/javascript”> <!-current_date = new Date(); document.write(current_date); //..> </script><br /> <br /> This code captures the current system date and formats it differently based on the selected browser, as illustrated in Figure 9.1. For example, Netscape 6 displays the Date object as:<br /> <br /> ANALYSIS<br /> <br /> Mon Nov 05 22:56:58 GMT-0600 (Central Standard Time) 2001<br /> <br /> This formatting will vary if you view the code on a Netscape browser running on a Macintosh computer. If you display the same JavaScript code using Microsoft Internet Explorer 6.0, you would get the following: Mon Nov 5 22:59:49 CST 2001<br /> <br /> OUTPUT FIGURE 9.1 The Date object is formatted differently depending upon the browser and the operating system.<br /> <br /> In this code, you will notice that we are using the keyword new before the Date object. When you use the new keyword in your code, you are instructing JavaScript to create a new Date object that you can then work with. When a Date object is created, it will always evaluate to an object data type and not a string or numeric value.<br /> <br /> Creating a Specific Date Object There also will be instances when you may want to ensure that you are working with a specific date. Instead of capturing the date from the user’s computer system, you can create a Date object to contain a specific date value. For example, if you want to perform calculations based on the number of days between today and Christmas, you could create a Date object that contains the date for Christmas as follows: Var Christmas = new Date(“December, 25 2002”);<br /> <br /> Date and Time Manipulation<br /> <br /> 311<br /> <br /> You can specify a date or time value for the Date object using any combination of the Date object arguments as outlined in Table 9.1. For example, when we specified the date for Christmas 2002, we actually passed the date value in the format of the month, day, year. In other words we used the arguments Month, dd, yyyy. TABLE 9.1<br /> <br /> Date Object Arguments<br /> <br /> Argument<br /> <br /> Description<br /> <br /> dd<br /> <br /> The day of the month specified with an integer value between 1 and 31<br /> <br /> hh<br /> <br /> The hour of the day specified with an integer value between 0 (representing midnight) and 23 (11:00 PM)<br /> <br /> mm<br /> <br /> The minute of the hour specified with an integer value between 0 and 59<br /> <br /> Month<br /> <br /> The full name of the month from “January” to “December”<br /> <br /> mth<br /> <br /> The month of the year specified as an integer value between 0 and 11 where 0 represents January and 11 represents December<br /> <br /> ms<br /> <br /> The millisecond portion of the time specified as an integer value between 0 and 999<br /> <br /> ss<br /> <br /> The second portion of the time specified as an integer value between 0 and 59<br /> <br /> yyyy<br /> <br /> A four-digit integer representing the year<br /> <br /> yy<br /> <br /> A two-digit integer between 00 and 99 representing the year<br /> <br /> You can use any combination of these arguments to specify a date value for the Date object. For example, you could use the following argument combination to specify the date and time: var date_time = new Date(yyyy, mth, dd, hh, mm, ss);<br /> <br /> You can create the actual date using this date and time format as illustrated: Var date_time = new Date(2001, 11, 05, 13, 15, 45);<br /> <br /> This date would be evaluated and then would display in an Internet Explorer browser window as: Mon Nov 5 13:15:45 CST 2001<br /> <br /> As you can see, no matter what format you use to pass the date to the Date object, the same information is always created for the object. If you simply write the contents of the Date object to the page, as demonstrated with Listing 9.1, the date information always displays the same based upon the settings of the particular browser.<br /> <br /> 9<br /> <br /> 312<br /> <br /> Day 9<br /> <br /> Obviously, you are not always going to be content to have that type of date output. Fortunately, the Date object includes several different methods that you can use to customize date and time output. For information about customizing the output from the Date object refer to the next two sections.<br /> <br /> Date Formatting As discussed earlier in this chapter, when you use the document.write statement to write the contents of the Date object to the browser window you get the date information formatted based on the default settings for each browser and operating system. For example, not only does Netscape Navigator display the date differently than Microsoft Internet Explorer, but the date information is also displayed differently on a Macintosh computer than a Microsoft Windows machine. That being the case, you are typically going to want to have more control over the actual display of the date information on the browser window. This can be accomplished by using the various methods available with the Date object for formatting the date portion. JavaScript provides methods for accessing each element of a date within a Date object and returning that particular value. For example, to return the month portion of a date you would use the getMonth() method. Listing 9.2 illustrates how to use the getMonth() method to return the numeric value that represents the current month.<br /> <br /> INPUT<br /> <br /> LISTING 9.2<br /> <br /> Returning the Current Month (formatdate.htm)<br /> <br /> <html> <head> <title>Current Month</title> </head> <body> <script language=”JavaScript” type=”text/javascript”> <!-var current_date = new Date(); month_value = current_date.getMonth(); document.write(“Current Month is “ + month_value); //--> </script> </body> </html><br /> <br /> The code in Listing 9.2 captures the current system date and then uses the getMonth() method to determine the current month portion of the date. The method returns a numeric value between 1 and 12 that represents the month of the year, as illustrated in Figure 9.2.<br /> <br /> ANALYSIS<br /> <br /> Date and Time Manipulation<br /> <br /> 313<br /> <br /> OUTPUT FIGURE 9.2 The getMonth() method of the Date object returns a numeric value representing the month of the year.<br /> <br /> Caution<br /> <br /> 9<br /> <br /> Be mindful of the fact that JavaScript counts the months in the year starting with 0. Therefore, January is actually 0 and December is 11. Make sure you remember this when you are converting dates.<br /> <br /> The Date object includes a specific method for each element of the date, as shown in Table 9.2. TABLE 9.2<br /> <br /> Methods for Getting Date Values<br /> <br /> Method<br /> <br /> Description<br /> <br /> getFullYear()<br /> <br /> Gets the year portion of the date as a four-digit number, such as 1998 or 2001<br /> <br /> getYear()<br /> <br /> Gets the year portion of the date as either a two or four-digit year depending on the browser used. Internet Explorer always returns a four-digit year. Netscape 4 and later subtracts 1900 from the date and returns the result. Earlier versions of Netscape truncate and return a two-digit year.<br /> <br /> getMonth()<br /> <br /> Gets the month of the year in the range of 0 to 11, where January is represented by 0 and December is 11<br /> <br /> getDate()<br /> <br /> Gets the numeric equivalent of the day of the month in the range of 1 to 31<br /> <br /> getDay()<br /> <br /> Gets the numeric equivalent of the day of the week in the range of 0 to 6 Sunday is 0 and Saturday is 6<br /> <br /> Be very careful about using the getYear() function. In fact, it is probably going to end up causing you more grief than anything. As mentioned in the table, when Netscape Navigator versions 4 or later see the getYear() method, the browser returns a year value by subtracting 1900 from the current year value. As you can imagine, this concept worked pretty well before the new millennium. For example, if the year is 1997 and you subtract 1900 you get a two-digit year of 97. But this type of date conversion no longer<br /> <br /> 314<br /> <br /> Day 9<br /> <br /> works. When you subtract 1900 from 2001 you get a value of 101. This being said, I would recommend just using the getFullYear() to return the year portion of any date. Another thing to keep in mind is the numeric range used to represent the months of the year and days of the week. Most of us are used to the concept of 12 months in a year and seven days in a week. That being said, the first month of the year is January and the last month is December. JavaScript uses a different numbering pattern in that both of these ranges start with the number zero, making the first value actually 0 instead of 1. If you want to return the actual name of the month or day you need to create code to convert the numeric value, as discussed in the following section.<br /> <br /> Converting the Numeric Day and Month Values More than likely when capturing the day of the week or even the month of the year you are going to want to display the text equivalent of the numeric value returned. As you noticed in Figure 9.1, each browser converts the Date object to display the text equivalent of the day of the week and the month of the year by default. Unfortunately if you decide to create your own date layout there are not any methods for the Date object to display the text equivalent of either the day of the week or the month value. Therefore, in order to display these values you must create your own JavaScript code to convert the numeric value returned by the getMonth() method and the getDay() method. Of course, as with anything you code, there are multiple ways that you can code the conversion from a numeric value to a text value. The most common methods would be to use an array of values as we discussed in Chapter 5, “An Introduction to Arrays.” Listing 9.3 illustrates how to use an array to convert the numeric value returned by the getDay() method to an actual day of the week.<br /> <br /> INPUT<br /> <br /> LISTING 9.3<br /> <br /> Converting the Day of the Week (convertday.htm)<br /> <br /> <html> <head> <title>Convert Day of the Week</title> </head> <body> <script language=”JavaScript” type=”text/javascript”> <!-var weekdays = new Array(7); weekdays[0] = “Sunday”; weekdays[1] = “Monday”; weekdays[2] = “Tuesday”; weekdays[3] = “Wednesday”; weekdays[4] = “Thursday”; weekdays[5] = “Friday”;<br /> <br /> Date and Time Manipulation<br /> <br /> LISTING 9.3<br /> <br /> 315<br /> <br /> continued<br /> <br /> weekdays[6] = “Saturday”; var current_date = new Date(); weekday_value = current_date.getDay(); document.write(“Today is “ + weekdays[weekday_value]); //--> </script> </body> </html><br /> <br /> This code takes the numeric value that is returned by the getDate() method and uses it as the index value for the weekdays array. Since the weekdays array contains 7 values from 0 to 6 the index values for the array elements match the numeric values returned by the getDate() method. For example, if today is Tuesday, the getDate() method returns a value of 2 and assigns that value to the weekday_value variable. The weekday_value variable is used as an index value to access the array. Since the element of the array with an index value of 2 is Tuesday, the JavaScript code displays that value on the page, as illustrated in Figure 9.3.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 9.3 You can create your own JavaScript code to convert the numeric value returned by getDate() to a day name.<br /> <br /> Of course you can create the same type of code to convert the numeric value returned by the getMonth() method into the name of the month. Again, keep in mind the fact that the getMonth() method returns a numeric value between 0 and 11 representing the month. Therefore, you need to make sure you design your code so that 0 represents January and 11 represents December, so that you will get the correct conversion of the numeric value returned by the method.<br /> <br /> 9<br /> <br /> 316<br /> <br /> Day 9<br /> <br /> Combining Date Values Up until now we have seen how easy it is to use methods for getting the elements of the Date object and to return a portion of the date, such as the day of the week, or even the current year. Obviously the main reason for manipulating the contents of the Date object would be to format the date value to match your own requirements for the date output. For example, you may want the ability to display the date in the format “month day, year,” or November 6, 2001. In order to display the date in this fashion you cannot just dump the contents of the Date object, as shown with Figure 9.1. However, making the date appear as you want is still quite simple. It simply requires using the concatenation operator (+) to join together the date elements you want to create your string. As you are aware, when you concatenate values together you are essentially creating one string of values from the original string. If you want to display the current date in the format mentioned above with the month followed by the day and year values, you need to concatenate those values together as illustrated in Listing 9.4.<br /> <br /> INPUT<br /> <br /> LISTING 9.4<br /> <br /> Combining Date Method Values (combinedate.htm)<br /> <br /> <html> <head> <title>Combine Date Values</title> </head> <body> <script language=”JavaScript” type=”text/javascript”> <!-var months = new Array(12); months[0] = “January”; months[1] = “February”; months[2] = “March”; months[3] = “April”; months[4] = “May”; months[5] = “June”; months[6] = “July”; months[7] = “August”; months[8] = “September”; months[9] = “October”; months[10] = “November”; months[11] = “December”; var current_date = new Date(); month_value = current_date.getMonth(); day_value = current_date.getDate(); year_value = current_date.getFullYear();<br /> <br /> Date and Time Manipulation<br /> <br /> LISTING 9.4<br /> <br /> 317<br /> <br /> continued<br /> <br /> document.write(“The current date is “ + months[month_value] + “ “ + day_value + “, “ + year_value); //--> </script> </body> </html><br /> <br /> 9 Notice with this code we use the array method that we discussed in the previous section to determine the month name that should be displayed as the current month name. When concatenating values together, remember you need to manually insert any spacing that you want in your string. Figure 9.4 shows the result of the code in Listing 9.4.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 9.4 Create the desired formatting for your date by combining the values returned by the date methods.<br /> <br /> As you can probably imagine, there are an infinite number of ways you can express a date, especially once you have retrieved the elements of the date that you need to display. In addition to working with the basic date portions of the Date object, don’t forget, the object also houses the time information for when the date is captured. You can combine any of the time information with your date statement for even more ways to customize your date statement. Let’s look at time values of the Date object in the next section.<br /> <br /> Time Formatting It seems quite odd to be looking in the Date object for time information, but that is where you will find it when working in JavaScript. As you remember from our discussion earlier in this chapter, JavaScript stores the information in the Date object as a millisecond value. In other words, the value of the Date object is a long number indicating the total number of milliseconds that passed between January 1, 1970 and the time when the Date object was created. So you see, although the object is called Date, it actually indicates a time value by specifying the amount of time since that date.<br /> <br /> 318<br /> <br /> Day 9<br /> <br /> Just like the date elements, you can also retrieve the individual elements of time from the Date object by using the associated methods. For example, you can use the getHours() to return the number of hours in the time portion of the Date object, as shown in Listing 9.5.<br /> <br /> INPUT<br /> <br /> LISTING 9.5<br /> <br /> Returning the Current Hour (ReturnHour.htm)<br /> <br /> <html> <head> <title>Current Hour</title> </head> <body> <script language=”JavaScript” type=”text/javascript”> <!-var current_date = new Date(); hour_value = current_date.getHours(); document.write(“The current hour is “ + hour_value); //--> </script> </body> </html><br /> <br /> The code in Listing 9.5 returns an integer value that represents the hour portion of the time. You will probably quickly notice that there are no AM or PM times. The value returned by the getHours() method is in a 24-hour clock format or what is commonly referred to as military time, as shown in Figure 9.5.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 9.5 Use of the getHours() method returns an integer value based on a 24-hour clock.<br /> <br /> Obviously there are other methods that you can use to return the other portions of the time. Table 9.3 lists the different methods that you can use with the Date object to return the time portions.<br /> <br /> Date and Time Manipulation<br /> <br /> TABLE 9.3<br /> <br /> 319<br /> <br /> Methods for Getting Time Values<br /> <br /> Method<br /> <br /> Description<br /> <br /> getHours()<br /> <br /> Gets the number of hours in the time. The method returns a value between 0 and 23, where 0 represents midnight and 23 represents 11:00 PM.<br /> <br /> getMinutes()<br /> <br /> Gets the number of minutes in the hour. The value returned is in the range of 0 to 59.<br /> <br /> getSeconds()<br /> <br /> Gets the number of seconds in the minute. The value returned is in the range of 0 to 59.<br /> <br /> getMilliseconds()<br /> <br /> Gets the number of milliseconds in the second. The value returned is in the range of 0 to 999.<br /> <br /> getTime()<br /> <br /> Returns a numeric value representing the total number of milliseconds since January 1, 1970 GMT.<br /> <br /> The getTime() method basically just returns the numeric value that JavaScript stores for the Date object. For example, if you create a Date object for the date 12/24/2001 the getTime() method returns a millisecond value of 1009173600000. This value is the same as the value that JavaScript uses to store the date. The time values returned from the Date methods can be combined with the same method used to create a custom date. For example, you can display an hour and minute combination by combining the values returned by the getHours() method with the getMinutes() method value. var current_date = new Date(); hour_value = current_date.getHours(); minute_value = current_date.getMinutes(); document.write(“The time is “ + hour_value + “:” + minute_value);<br /> <br /> Make sure that when you combine the time values you insert the colon (:), or whatever symbol you want to use to differentiate between an hour, minute, or second value. Of course all of these times will be based upon a 24-hour clock, so if it is 8:30 PM, the time displays as 20:30. The only way to remedy this is to manually convert the time value before displaying it.<br /> <br /> Converting to an AM/PM Time Frame Depending upon where you live and how you were taught to tell time, you probably will not be content to get a 24-hour clock format for the time. Even if you like stating it as 1900 hours, visitors to your site most likely would prefer seeing the time displayed in the<br /> <br /> 9<br /> <br /> 320<br /> <br /> Day 9<br /> <br /> more traditional fashion of 7:00 PM. That being said, JavaScript obviously does not provide a default solution for doing this. If you want to display the AM and PM times, you need to manually code a method to convert the value returned by the getHours() method to the desired format. If you are familiar with the 24-hour clock concept, you know that the hours start counting at midnight with 0. Noon is considered to be 12:00 which is followed by 13:00 (commonly referred to as 1:00 PM). With a 24-hour clock, each hour gets progressively higher until 23:00, or 11:00 PM. Of course, at midnight the counting starts over. Listing 9.6 illustrates a simple method to taking the current time and converting it to display in the AM/PM format.<br /> <br /> INPUT<br /> <br /> LISTING 9.6<br /> <br /> Returning the Current Hour (ConvertTime.htm)<br /> <br /> <html> <head> <title>Convert Time</title> </head> <body> <script language=”JavaScript” type=”text/javascript”> <!-var var var var var<br /> <br /> current_date = new Date(); hour_value = current_date.getHours(); minute_value = current_date.getMinutes(); second_value = current_date.getSeconds(); AMorPM = “AM”;<br /> <br /> if (hour_value > 12) { hour_value -= 12; AMorPM = “PM”; } document.write(“The current time is “ + hour_value + “:” + minute_value + “:” + second_value + “ “ + AMorPM); //--> </script> </body> </html><br /> <br /> The code in Listing 9.6 takes the numeric value returned by the getHours() method and subtracts 12 from it, if the value is larger than 12 and sets the AMorPM variable to PM. For example, if the time is 17:30 the code converts it to 5:30 PM, as illustrated in Figure 9.6.<br /> <br /> ANALYSIS<br /> <br /> Date and Time Manipulation<br /> <br /> 321<br /> <br /> OUTPUT FIGURE 9.6 You can create custom JavaScript code to convert from 24-hour clock to AM/PM times.<br /> <br /> 9<br /> <br /> This code works well for displaying the current time information on the browser page. But there is one small issue. Since the values that are returned are all integer values it only displays single digits for values less that 10. This can make a time like 10:05 look a little odd when it displays as 10:5. To fix this problem, you can add additional if statements that look at the value returned by the getMinutes() and getSeconds() methods, and adds a zero to the front if the value is less than 10, as shown in Listing 9.7.<br /> <br /> INPUT<br /> <br /> LISTING 9.7<br /> <br /> Adjusting Time Values under 10 (AdjustTime.htm)<br /> <br /> <html> <head> <title>Convert Time</title> </head> <body> <script language=”JavaScript” type=”text/javascript”> <!-var var var var var<br /> <br /> current_date = new Date(); hour_value = current_date.getHours(); minute_value = current_date.getMinutes(); second_value = current_date.getSeconds(); AMorPM = “AM”;<br /> <br /> if (hour_value > 12) { hour_value -= 12; AMorPM = “PM”; } if (hour_value == 0) { hour_value = 12; AMorPM = “AM”; } if (second_value < 10) {<br /> <br /> 322<br /> <br /> Day 9<br /> <br /> LISTING 9.7<br /> <br /> continued<br /> <br /> second_value = “0” + second_value; } if (minute_value < 10) { minute_value = “0” + minute_value; } document.write(“The current time is “ + hour_value + “:” + minute_value + “:” + second_value + “ “ + AMorPM); //--> </script> </body> </html><br /> <br /> If you look at the code in Listing 9.7, you will see that the if statements check the value of the second_value and minute_value variables. If either of the variables contains a value less than 10, a value of zero is placed in front of that value. Therefore, your times will all display in the format HH:MM:SS.<br /> <br /> ANALYSIS<br /> <br /> Working with Time Zones As we have already talked about, each computer system maintains the time zone information in relation to GMT. Of course, the time zone setting is only accurate if the user has properly set it. You may find the need to capture the user’s time zone in order to perform different time calculations, such as determining the amount of time before a specific event transpires. In order to do this you can use the getTimezoneOffset() method of the Date object to determine the number of minutes between the current time zone and GMT. Listing 9.8 illustrates how to use the getTimezoneOffset() method to determine the number of minutes from GMT for the user’s machine.<br /> <br /> INPUT<br /> <br /> LISTING 9.8<br /> <br /> Determining Time Difference from GMT (gettimezone.htm)<br /> <br /> <html> <head> <title>Determine Time Zone</title> </head> <body> <script language=”JavaScript” type=”text/javascript”> <!--<br /> <br /> Date and Time Manipulation<br /> <br /> LISTING 9.8<br /> <br /> 323<br /> <br /> continued<br /> <br /> var current_date = new Date(); var current_timezone = current_date.getTimezoneOffset(); document.write(“Your time zone is “ + current_timezone + “ minutes from GMT”); //--> </script> </body> </html><br /> <br /> Again this code is quite simple, but it illustrates how easy it is to determine the user’s time zone, as shown in Figure 9.7. If you want to have an event transpire at a specific time based on your time zone, you can use the getTimezoneOffset() method to determine the user’s time zone offset, and then use the offset for your time zone to make the comparisons.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 9.7 You can determine the actual difference in time between your location and the user by working with the time zone offset value.<br /> <br /> Converting Date and Time Formats So far in this chapter we have talked about the process of retrieving a date or a portion of a date value from a particular Date object. Of course, retrieving the date and time information is not the only process that you can or will want to do. Sooner or later you will want to modify the contents of a Date object based on particular date calculations, such as calculating a shipping date 30 days away, or determining a final payment date. Whatever the reason, there are many options for working with the date information to achieve the desired results. Typically the best method of changing a date is to create an initial Date object and then perform the desired calculations on the date. Once you have the new date, you can make the appropriate modifications to the Date object by using the Date object method for<br /> <br /> 9<br /> <br /> 324<br /> <br /> Day 9<br /> <br /> changing the object. These methods, shown in Table 9.4, allow you to change all portions of the Date object, including the time elements. TABLE 9.4<br /> <br /> Methods for Setting Date Object<br /> <br /> Method<br /> <br /> Description<br /> <br /> setDate(dd)<br /> <br /> Accepts an integer value between 1 and 31 to specify the actual day of the month<br /> <br /> setFullYear(yyyy)<br /> <br /> Accepts a four-digit number that specifies the year, such as 1998 or 2002<br /> <br /> setHours(hh)<br /> <br /> Accepts an integer value between 0 and 23 specifying the hour<br /> <br /> setMilliseconds(ms)<br /> <br /> Accepts an integer value between 0 and 999 specifying the number of milliseconds<br /> <br /> setMinutes(mm)<br /> <br /> Accepts an integer value between 0 and 59 specifying the number of minutes<br /> <br /> setMonth(mth)<br /> <br /> Accepts an integer value between 0 (January) and 11 (December) specifying the month number<br /> <br /> setSeconds(ss)<br /> <br /> Accepts an integer value between 0 and 59 specifying the number of seconds<br /> <br /> setTime(ms)<br /> <br /> Sets the time by passing a number representing the number of milliseconds that have passed since January 1, 1970 GMT<br /> <br /> setYear(yy)<br /> <br /> Accepts a two-digit or four-digit number that specifies the year<br /> <br /> Note<br /> <br /> If you use the setYear() method, you should always specify a four-digit year to avoid any confusion with the actual date. The two-digit date only works for dates between 1900 and 1999. If you type a 01 for 2001, JavaScript assumes you are referring to the date 1901 and treats it as such. By always typing a four-digit date you avoid any potential problems.<br /> <br /> As you have noticed, all of the methods in Table 9.4 require an argument containing the value to be passed to the Date object. You should recognize these arguments as the same arguments that we looked at in Table 9.1. For example, to change the day of the month you need to use the setDate() method with the dd argument. For example, you can change the year of the Date object by assigning a new year with the setYear() method as illustrated in the following lines of code that retrieve the new year from the user and assign it to the Date object.<br /> <br /> Date and Time Manipulation<br /> <br /> 325<br /> <br /> var user_date = new Date(); var new_year = prompt(“Enter Year”); user_date.setYear(new_year);<br /> <br /> As you can see it is fairly easy to modify the value of a Date object at any time simply by using the methods outlined in Table 9.4. The modifications to the date can either come from user input, as shown in the lines of code above, or by calculations performed on the date information. For example, you may want to add 45 days to a date. Of course the types of calculations that can be performed on a date are unlimited, as we will discuss in the following section.<br /> <br /> Date Calculations So far in this chapter we have looked at creating the Date object, retrieving information from the Date object, and modifying the value of the Date object. Quite often you will want to perform some type of calculation on the date values before writing the results to the Date object. Since the Date object stores the entire date and time information as a numeric value it makes the value quite easy to work with. The complexity comes with the fact that the numeric value is stored in milliseconds. Therefore, in order to calculate the amount of time between dates you need to convert the numeric value accordingly. For example, there are 1000 milliseconds in just one second. Table 9.5 illustrates the conversion rates for each element of the Date object. TABLE 9.5<br /> <br /> Converting Date Values<br /> <br /> Date Value<br /> <br /> Conversion<br /> <br /> Second<br /> <br /> 1000 milliseconds<br /> <br /> Minute<br /> <br /> 60 * 1000 = 60,000 milliseconds<br /> <br /> Hour<br /> <br /> 60 * 60 * 1000 = 3,600,000 milliseconds<br /> <br /> Day<br /> <br /> 24 * 60 * 60 * 1000 = 86,400,000 milliseconds<br /> <br /> Week<br /> <br /> 7 * 24 * 60 * 60 * 1000 = 604,800,000<br /> <br /> Month<br /> <br /> (days in month) * 7 * 24 * 60 * 1000<br /> <br /> year<br /> <br /> 365 * 24 * 60 * 60 * 1000 = 31,536,000,000 milliseconds<br /> <br /> As show in Table 9.5, it doesn’t take much to create some very large Date values. In fact, each year that passes is equal to 31,536,000,000 milliseconds.<br /> <br /> 9<br /> <br /> 326<br /> <br /> Day 9<br /> <br /> You will want to keep these conversions in mind as you start manipulating different date values to ensure you return the appropriate results. For example, Listing 9.9 illustrates how you can prompt the user for a future date and determine the number of years, months, and days between the current date and the future date.<br /> <br /> INPUT<br /> <br /> LISTING 9.9<br /> <br /> Comparing Dates (setdate.htm)<br /> <br /> <html> <head> <title>Find Time Until Future Date</title> </head> <body> <script language=”JavaScript” type=”text/javascript”> <!-var user_date = prompt(“Enter a future date: “) var var var var var var<br /> <br /> user_date = Date.parse(user_date); today_date = new Date(); diff_date = user_date - today_date; num_years = diff_date/31536000000; num_months = (diff_date % 31536000000)/2628000000; num_days = ((diff_date % 31536000000) % 2628000000)/86400000;<br /> <br /> document.write(“Number of years: “ + Math.floor(num_years) + “<br>”); document.write(“Number of months: “ + Math.floor(num_months) + “<br>”); document.write(“Number of days: “ + Math.floor(num_days) + “<br>”); //--> </script> </body> </html><br /> <br /> Notice how in Listing 9.9 we had to take the total date value and divide it by the total number of milliseconds in a year to determine how many years were between the current date and the specified future date. Next we take the remainder of the year calculation and determine the number of months, and then the number of days. The code displays the total amount of time between the current date and the future date broken into the number of years, months, and days between the dates, as shown in Figure 9.8.<br /> <br /> ANALYSIS<br /> <br /> You should pay close attention to the fact that the code in Listing 9.9 accepts a date value entered by the user and then uses the parse() method to convert the date value to a Date object. The parse() method definitely comes in handy when you want to receive a date from a user. By using this method you can take any date string and convert it into a Date object. The following section talks more about this method.<br /> <br /> Date and Time Manipulation<br /> <br /> 327<br /> <br /> OUTPUT FIGURE 9.8 You can perform calculations to determine the specific amount of time between two dates.<br /> <br /> Converting Between Strings and Dates Obviously when you prompt a user for a date you are going to get some form of a date string. For example, a user could enter the date for Thanksgiving 2001 as 11/22/2001, November 22, 2001, or even Nov. 22, 2001. Of course you could always ask for each individual portion of the date, such as the year, month, and day and then piece the date together using the set methods discussed in Table 9.4, but that is cumbersome. The easier method is to accept the date string and then convert the string into a Date object. This can be accomplished by using the parse() method. The parse() method accepts any valid date string and converts it into a standard Date object. We used this method in Listing 9.9 when we converted the string entered by the user. We needed to convert the date value to a Date object so that we could calculate the difference between that date and the current date, or the number of milliseconds between the two dates.<br /> <br /> Summary In this chapter we looked at the JavaScript Date object, and how dates and times are stored as a millisecond value. The value is calculated as the number of milliseconds since January 1, 1970. Because JavaScript stores date and time information in one object, you can extract the elements of the object that you need to perform the desired calculations by using any of the different get methods available with the Date object. You can also modify the contents of a Date object using the various set methods. For example, you may want to change the date to reflect a date 30 days in the future. You can do this by using the setDate() method. Finally, we explored how the millisecond values can be manipulated to perform desired calculations, such as determining the amount of time between two dates. Because there are 1000 milliseconds within just one second, the size of the number stored by a Date object is typically quite large.<br /> <br /> 9<br /> <br /> 328<br /> <br /> Day 9<br /> <br /> Workshop In the workshop today we will look at some Q&A issues related to working with the Date object and we will use the questions and exercises to review what you have learned in this chapter about the Date object.<br /> <br /> Q&A Q. When do I need to use the new keyword with the Date object? A. The new keyword always precedes the Date object. The object is declared in the form: var objectname = new Date(). Q. Why does my 02 appear as 1902, instead or 2002, in the browser window? A. Most browsers assume that a two-digit date represents a date between 1900 and 1999. If a value of 01 is specified for a date, it will be interpreted as 1901. You need to specify a four-digit year value to avoid year conversion errors. Q. When I convert 24-hour clock times to AM and PM I get 00 for times between midnight and 1:00 AM, how do I fix that? A. When working with a 24-hour clock, times between midnight and 1:00 AM are displayed with the hour as 00. Therefore, 12:15 AM will actually appear as 00:15. In order to make this more readable, you need to convert the hour value to 00. You can do this with the following code: if (hour_value == 0) { hour_value = 12; AMorPM = “AM”; }<br /> <br /> Quiz 1. What is the basic unit used for storing date and time values with the Date object? 2. What is the range of integer values used to represent the months of the year? 3. Why worry about custom formatting a date before displaying it on the browser window? 4. What is a 24-hour clock format and how would you convert it to AM/PM?<br /> <br /> Quiz Answers 1. All date and time information is stored in milliseconds. 2. The 12 months of the year are specified from 0, representing January, to 11 for December.<br /> <br /> Date and Time Manipulation<br /> <br /> 329<br /> <br /> 3. Each browser displays the Date object using a different format. Typically you want your date and time information to display consistently on all browsers. In order to do so, you need to custom format the date output using the set methods for the Date object. 4. When you request an hour from a Date object it returns the value based upon a 24hour clock. The 24-hour clock starts counting with 0 at midnight and 11:00 PM returns as 23. In order to convert to an AM/PM time you can subtract 12 from all times that are 13 and over, and specify PM for those times.<br /> <br /> Exercises 1. Create an HTML page to count down the number of days until a special event, such as Christmas, or a special vacation. 2. Create an HTML page to determine how old an individual is based on his birthdate.<br /> <br /> 9<br /> <br /> WEEK 2<br /> <br /> DAY<br /> <br /> 10<br /> <br /> Events and Event Handling Whenever anything occurs within a browser window an event is created. An event can be as simple as a mouse cursor moving across the screen or it can be a result of opening a new page. In this chapter we will look at the different types of events that can occur within the browser window and how you can monitor these events to determine when your code should perform certain actions. We also will look at performing the most popular type of event handling these days—the creation of image rollovers. This chapter will teach you: • What an event is • Types of events • How to handle events • How to create mouse-over effects<br /> <br /> 332<br /> <br /> Day 10<br /> <br /> Understanding Events Whenever anything happens, your JavaScript code changes the value of an object; or, for example, even when an error occurs, an event takes place. For instance, if the user clicks a button on a page, a Click event occurs for that button. If your code updates the value of a form field, a Change event occurs. By interacting with the HTML code on the page, you have the ability to capture the events that occur in the browser window and perform different actions based upon the specific events. Probably the most commonly captured event is the Click event. This event occurs whenever a single mouse click occurs on the browser window. Of course, you may not care about all mouse clicks, but if the user clicks on a Submit button at the bottom of the form you definitely want to be aware of that event. As a conscientious programmer you want to capture the Click event from a Submit button to verify the data entered on the form. This ensures the values are within the appropriate ranges before the information is submitted to the server. For example, if a form field must contain a value between 1 and 10, you can verify the contents of the field and display a message for the user to correct the value if it is incorrect. When you create JavaScript code to capture an event you are writing what is commonly referred to as an event handler. The event handler specifies what actions you want to perform when the particular event occurs on the page. An event handler typically consists of two pieces as described in the following section.<br /> <br /> NEW TERM<br /> <br /> Creating an Event Handler Events can occur all day long on your HTML pages, but if you do not create code to capture and handle the events, the events are meaningless. For example, you can place a button on a form, but if you do not create the code to recognize the fact that the button has been depressed, the vistor to your site can press the button, but there will be no response. The coding that is done to handle the event is the event handler. Although an HTML object typically triggers an event, HTML cannot handle an event on its own. In order to respond to an event you must use some other language, such as JavaScript. What we are going to do is create event handlers that respond when specific properties are set by HTML objects. For example, the following HTML tag includes an onClick event handler to capture the clicking of the button by the user. <input type=”button” value=”Push Button” name=”Button1” onclick=”window.alert(‘Button Pressed’);”><br /> <br /> Events and Event Handling<br /> <br /> 333<br /> <br /> There are basically three types of event handlers that you can create. Each type requires the capture of an HTML object property, as outlined in Table 10.1. TABLE 10.1<br /> <br /> Types of Event Handlers<br /> <br /> Type<br /> <br /> Description<br /> <br /> Statement Event Handler<br /> <br /> As shown in the sample code above, the HTML tag property value is assigned a JavaScript statement that executes when the event occurs.<br /> <br /> Function Event Handler<br /> <br /> The HTML tag property is set to call a specific JavaScript function when the corresponding event occurs such as, onclick=”Button_Clicked();”<br /> <br /> JavaScript Capture Events<br /> <br /> Used to create generic JavaScript code that executes each time a specific event occurs, such as loading a particular page.<br /> <br /> Each of these event handling methods is discussed in detail in the following sections.<br /> <br /> Handling an Event with a JavaScript Statement When you want to perform a simple action, such as display a message, when an event occurs you can place the entire event handler within the tag of the HTML object. For example, the following code displays an alert message box when the onclick property is triggered for the Input button. <input type=”button” value=”Push Button” name=”Button1” onclick=”window.alert(‘Button Pressed’);”><br /> <br /> You add the event handler to your HTML code as part of the particular object’s tag. Each event handler must be made up of two specific elements: the event handler attribute and the value of the attribute. In the above sample code, the event handler is onclick and the value of the attribute is window.alert(‘Button Pressed’); as soon as the button is pressed, the alert message displays in the browser window, as shown in Figure 10.1. FIGURE 10.1 When the specified event is triggered the corresponding JavaScript code executes.<br /> <br /> 10<br /> <br /> 334<br /> <br /> Day 10<br /> <br /> Be sure to notice that the JavaScript code that executes for the event is placed in double quotes within the HTML tag. HTML does not know how to decipher the JavaScript code. By placing the code in quotes, it is ignored by HTML and executes as JavaScript code.<br /> <br /> Tip<br /> <br /> Remember when you need to put quotes around values that are already within quotes you need to differentiate between the two types of quotes by alternating between double and single quotes. For example, in the event handler code onClick=”window.alert(‘Button Pressed’);” the double quotes are used to specify the event handler code and the single quotes are used to indicate the text to display on the alert box. You can use the quotes in any order, either double and then single or vice versa as long as you make sure they match up correctly.<br /> <br /> You can use this type of event handling when you only have one JavaScript statement to execute. If you have multiple statements, it is better to call a JavaScript function, described in the following section.<br /> <br /> Handling Events with JavaScript Function Calls So far we have looked at launching a JavaScript command directly from an HTML object tag. When you have more than one JavaScript statement to launch, it makes more sense to call a JavaScript function from the HTML object tag. In other words, you still use the HTML object property to determine when the event has occurred by setting the value of the property to call a JavaScript function, as shown in Listing 10.1.<br /> <br /> INPUT<br /> <br /> LISTING 10.1<br /> <br /> Calling a JavaScript Function for an Event (eventscript.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function CheckValue() { if ((document.golf.balls.value >= 1) && (document.golf.balls.value <= 10)) { var total_amount = document.golf.balls.value * 25; window.alert(“Total Cost is: “ + “$” + total_amount); } else window.alert(“Type a value between 1 and 10”); }<br /> <br /> Events and Event Handling<br /> <br /> LISTING 10.1<br /> <br /> 335<br /> <br /> continued<br /> <br /> //..> </script> <title>Golf Balls</title> </head> <body> <form name=”golf” method=”POST” > <font size=”4”>Golf Balls</font><br> $25.00/dzn.    Quantity <input type=”text” name=”balls” size=”5”>    <input type=”button” value=”Order” name=”Order” OnClick=”CheckValue()”></p> </form> </body> </html><br /> <br /> In Listing 10.1, the onClick property triggers the CheckValue function if the Order button is pressed. The CheckValue function verifies that the value specified in the Quantity text box is within the appropriate range and then displays the total cost, as illustrated in Figure 10.2.<br /> <br /> OUTPUT FIGURE 10.2 By capturing the events with a form you can verify that the form contains the appropriate values before submitting the form information to the server.<br /> <br /> Button events are very commonly used to validate values on a form, as shown in Listing 10.1. The form validation can be done with either an onClick event, or an onSubmit event triggered from a Submit button being pressed on a form. There are some objects though that do not have HTML tags. Events triggered by these objects require a different type of event handling, as outlined in the following section.<br /> <br /> Capturing Events Directly Within JavaScript The final method for capturing object events is to create JavaScript code that watches for a particular event to occur. This type of coding works best for objects (such as the Document object) that do not have HTML tags, although you can use this method for any<br /> <br /> 10<br /> <br /> 336<br /> <br /> Day 10<br /> <br /> event. Keep in mind this method captures all occurrences of the specified event for the object. In order to use this type of method, you need to specify the event handler code statement within your JavaScript code. The event handler is coded as follows: objectname.eventname = eventhandler<br /> <br /> The objectname value is the name of the object for which you want to capture events. If you want to capture all events of a type that occur on the entire page, you will want to specify the Document object. When you specify the eventname value you must place the word on in front of the event and the entire name must be all lowercase. For example, to track the Load event, you would type onload. The event hander is simply a call to a JavaScript function to handle the event. This resembles the function calls we made in the previous section. The only major difference is that you do not place the parentheses after the function name. It also means that you cannot pass data values into the function. The event handler is coded as follows: Window.onresize = DisplayMsg;<br /> <br /> The code in Listing 10.2 illustrates how to create an event handler for any events of a specified type. For example, the sample code watches for the browser window to be resized, and then displays a message.<br /> <br /> INPUT<br /> <br /> LISTING 10.2<br /> <br /> Capturing Events Within JavaScript (resizescript.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-window.onresize = DisplayMsg; function DisplayMsg() { window.alert(“You just resized the window.”); } //..> </script> <title>Window Resizing</title> </head> <body> </body> </html><br /> <br /> Events and Event Handling<br /> <br /> 337<br /> <br /> With the code in Listing 10.2, the DisplayMsg function launches each time the browser window is resized, and then displays a message indicating what occurred, as shown in Figure 10.3. Notice that we are monitoring the Window object for the event to occur. We will discuss more about which objects trigger the events you want to monitor later in this chapter.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 10.3 The resize event handler is triggered within your JavaScript code each time that type of event occurs.<br /> <br /> 10<br /> <br /> Types of Events As we have already seen so far, there are several different types of events for which you can create event handlers. All of the events available in JavaScript mirror the HTML events. Although JavaScript does not allow you to script for all events that may occur on the page, you will see in Table 10.2 that you can capture pretty much all the events you need to make your code run effectively. One thing you are going to notice with events is that each object triggers a series of events, although the same events can be triggered by multiple objects. For example, the Click event is triggered by several different objects including the Document, BUTTON, RadioButton, and TABLE objects. You create an event handler for each of the events that JavaScript supports by using the keyword on before the name of the event, as illustrated in Table 10.2. TABLE 10.2<br /> <br /> JavaScript Events<br /> <br /> Event<br /> <br /> Event Handler<br /> <br /> Description<br /> <br /> Abort<br /> <br /> onabort<br /> <br /> Occurs when loading of a page or image is canceled<br /> <br /> Blur<br /> <br /> onblur<br /> <br /> Occurs when the focus moves to a different object<br /> <br /> Change<br /> <br /> onchange<br /> <br /> Occurs when a field on a form is modified<br /> <br /> Click<br /> <br /> onclick<br /> <br /> Occurs when an object is clicked with the mouse<br /> <br /> DblClick<br /> <br /> ondblclick<br /> <br /> Occurs when an object is double-clicked with the mouse<br /> <br /> 338<br /> <br /> Day 10<br /> <br /> TABLE 10.2<br /> <br /> continued<br /> <br /> Event<br /> <br /> Event Handler<br /> <br /> Description<br /> <br /> Error<br /> <br /> onerror<br /> <br /> Occurs when an error occurs on the page. (We are not going to deal with the Error object in this chapter. Error handling is covered extensively in Chapter 15.)<br /> <br /> Focus<br /> <br /> onfocus<br /> <br /> Occurs when an object receives focus<br /> <br /> KeyDown<br /> <br /> onkeydown<br /> <br /> Occurs when a key is pressed on the keyboard<br /> <br /> KeyPress<br /> <br /> onkeypress<br /> <br /> Occurs when a key is pressed and then released on the keyboard<br /> <br /> KeyUp<br /> <br /> onkeyup<br /> <br /> Occurs when a pressed key is released on the keyboard<br /> <br /> Load<br /> <br /> onload<br /> <br /> Occurs when a page or image loads<br /> <br /> MouseDown<br /> <br /> onmousedown<br /> <br /> Occurs when a mouse button is pressed<br /> <br /> MouseMove<br /> <br /> onmousemove<br /> <br /> Occurs when the mouse pointer is moved<br /> <br /> MouseOut<br /> <br /> onmouseout<br /> <br /> Occurs when the mouse pointer moves off of an object<br /> <br /> MouseOver<br /> <br /> onmouseover<br /> <br /> Occurs when the mouse pointer moves over an object<br /> <br /> MouseUp<br /> <br /> onmouseup<br /> <br /> Occurs when a pressed mouse button is released<br /> <br /> Move<br /> <br /> onmove<br /> <br /> Occurs when the browser window is moved<br /> <br /> Reset<br /> <br /> onreset<br /> <br /> Occurs when the Reset button is pressed on a form<br /> <br /> Resize<br /> <br /> onresize<br /> <br /> Occurs when the browser window is resized<br /> <br /> Select<br /> <br /> onselect<br /> <br /> Occurs when text is highlighted on the page<br /> <br /> Submit<br /> <br /> onsubmit<br /> <br /> Occurs when the Submit button is pressed on the form<br /> <br /> Unload<br /> <br /> onunload<br /> <br /> Occurs when the page unloads<br /> <br /> Note<br /> <br /> The events listed in Table 10.2 reflect those that are available and supported by both JavaScript and JScript, ensuring that they are valid in both Internet Explorer and Netscape Navigator. You will find that there are additional event handlers that you can use that are only supported by one of the two languages, such as onCut, which is an Internet Explorer-only event handler. If you are creating code specific to one browser type, you can use the additional events supported by that browser. Refer to Chapter 8, “The Browser Issue,” for more information on browser compatibility issues.<br /> <br /> Events and Event Handling<br /> <br /> 339<br /> <br /> How to Handle Events Earlier in the chapter we looked at the different methods you can use for creating event handler code. If you remember we looked at how the event handler can be handled as either part of the HTML object tag or as a separate JavaScript function. Of course the type of event handler you use will vary based on the type of event you are handling. We now need to look at handling specific event types in more detail and the corresponding objects that trigger each event.<br /> <br /> Monitoring Form Changes As we discussed previously, events are very useful when working with forms. By capturing events related to the form, you can determine when the user changes a field or presses a button. In Listing 10.1 we used the Click event to determine when a button had been pressed. If you remember, after the button was pressed we checked the value of the text box to ensure that it contained a valid value. This same process can be accomplished by monitoring a Submit event triggered by a Submit button. We will talk more about using the Click event as we deal with mouse actions later in the chapter. Of course the use of the Click or Submit event requires waiting for the user to press a button before any type of validation can be performed on the values in the form fields. This can become onerous if you have a long form. A better method would be to be able to validate the values in a form field each time the field changes. This type of event handling can be accomplished using the Change event. You can use the Change event with the form fields that accept values including text boxes, check boxes, and radio buttons. Each time the value of one of these fields changes the Change event is triggered. You can create code to validate the specified form field value, as shown in Listing 10.3.<br /> <br /> INPUT<br /> <br /> LISTING 10.3<br /> <br /> Capturing Change Events (changeevent.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function CheckValue(field_change) { switch(field_change.name) {<br /> <br /> 10<br /> <br /> 340<br /> <br /> Day 10<br /> <br /> LISTING 10.3<br /> <br /> continued<br /> <br /> case “balls” : if ((field_change.value <= 0) || (field_change.value >= 10)) { window.alert(“Type a value between 1 and 10”); } break; case “tees”: if ((field_change.value <= 0) || (field_change.value >= 20)) { window.alert(“Type a value between 1 and 20”); } break; case “gloves”: if ((field_change.value <= 0) || (field_change.value >= 5)) { window.alert(“Type a value between 1 and 5”); } break; } } //..> </script> <title>Golf Balls</title> </head> <body> <form name=”golf” method=”POST” > <font size=”4”>Golf Balls</font><br>$25.00/dozen      Quantity <input type=”text” name=”balls” size=”5” onchange=”CheckValue(this)”>    <p> <font size=”4”>Golf Tees</font><br>$1.00/hundred    Quantity <input type=”text” name=”tees” size=”5” onchange=”CheckValue(this)”></p> <p>Golf Glove<br>$15.00/each        Quantity <input type=”text” name=”gloves” size=”5” onchange=”CheckValue(this)”></p> <p> </p> <p>                 <input type=”button” value=”Order” name=”Order” ></p> </p> </form> </body> </html><br /> <br /> Events and Event Handling<br /> <br /> 341<br /> <br /> The code in Listing 10.3 monitors the three different text fields to determine when the value changes. If the value changes the Change event is triggered and the CheckValue function determines if the value specified in the text box falls within the range of acceptable values.<br /> <br /> ANALYSIS<br /> <br /> Notice that we are calling the same function for the Change event on each text box. The CheckValue function determines which text box was modified and executes the appropriate code. In order to know which text box has been modified, we are using the this keyword to pass the reference to the object to the function. Once the reference is passed to the CheckValue function the name value of the text box object is checked to determine which text box was altered. We then use the Switch statement to determine which statements to execute based on the text box that was modified. Once the appropriate code executes, an alert message displays if the value of the text box is not within the appropriate range, as illustrated in Figure 10.4. FIGURE 10.4 You can use the Change event to monitor changes to specific form fields.<br /> <br /> Note<br /> <br /> We discussed forms in Chapter 6, “HTML Forms and the String Object.” For more information on form fields refer to that chapter.<br /> <br /> Using the this Keyword with Event Handlers When creating event handlers you will need to know which object triggered a specific event. Since multiple objects can trigger the same event, you need a means of capturing that information. This can be accomplished using the this keyword, as we did in Listing 10.3. The this keyword allows you to pass a reference to an object as an argument for a function. For example, in Listing 10.3 we have the following code for each text box: onchange=CheckValue(this)<br /> <br /> When the Change event is triggered for a particular text box the CheckValue function is called and it receives a reference to the actual object that triggered the Change event. You then can access properties of the object. For example, we are using the Name property to determine which object was triggered.<br /> <br /> 10<br /> <br /> 342<br /> <br /> Day 10<br /> <br /> Working with Keyboard Events When dealing with user input you may want to monitor what keys are pressed on the keyboard. Each time you press a key on the keyboard there are actually a sequence of three different events that are triggered. When you press down the key, a KeyDown event is triggered; as the key is released, a KeyUp event triggers; and finally, after you have pressed a key a KeyPress event triggers. These events occur for every key that is pressed. You can use these three keyboard events for monitoring input in a specific Form object, such as a text box, or any other location on the Document object. For example, Listing 10.4 monitors any key presses that occur on the Web page and then displays a message indicating that a key was pressed.<br /> <br /> INPUT<br /> <br /> LISTING 10.4<br /> <br /> Capturing Keyboard Events (keypress.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-document.onkeypress = DisplayMsg; function DisplayMsg() { window.alert(“You just pressed a key.”); } //..> </script> <title>Keyboard Event</title> </head> <body> </body> </html><br /> <br /> If you look closely at the code in Listing 10.4, you will notice that the alert message displays for every key that is pressed. If you are only concerned with the keyboard events within a specific form field, you again can monitor only that specific object for keyboard events.<br /> <br /> ANALYSIS<br /> <br /> Note<br /> <br /> Keep in mind that you can rely only on receiving these events when keys with values on the ASCII table are pressed. This includes all alphanumeric keys, the Enter key, and the spacebar. Typically you will not receive events from pressing function keys, arrow keys, or any other navigational key.<br /> <br /> Events and Event Handling<br /> <br /> 343<br /> <br /> Of course, just knowing a key has been pressed may not be adequate. It also comes in handy to know which key was actually pressed. In order to do that we need to use the Event object so that we can get more information about the event that transpired.<br /> <br /> Using the Event Object Both the Netscape and Internet Explorer object models provide an Event object which stores information about the current event. JavaScript can access this object at anytime in order to gather more information about an event. Since it is an object, we can access any of the properties associated with the object to get more information about the event— that’s the cool part of the deal. Unfortunately Microsoft and Netscape do not support the same properties for this object. Because of this fact, in order for our script to work on both browsers we must create browser-specific code for each browser, as shown in Listing 10.5.<br /> <br /> Note INPUT<br /> <br /> For specific information about creating cross-browser code refer to Chapter 8.<br /> <br /> LISTING 10.5<br /> <br /> Determining Which Key Was Pressed (capture keypress.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-document.onkeypress = DisplayMsg; function DisplayMsg(key_event) { if (document.all) //Checks for IE 4.0 or later { document.form1.text2.value = String.fromCharCode(event.keyCode); } else if (document.getElementById) //checks for Netscape 6 or later { document.form1.text2.value = String.fromCharCode(key_event.which); } else if (document.layers) //Checks for Netscape 4 { document.form1.text2.value = String.fromCharCode(key_event.which);<br /> <br /> 10<br /> <br /> 344<br /> <br /> Day 10<br /> <br /> LISTING 10.5 continued } } //..> </script> <title>Capture Key Pressed</title> </head> <body> <form name=”form1”> <b>Type value in field:                  See what you typed:</b><br> <input type = “text” name = “text1” onkeypress=”DisplayMsg(event)” size=”20”>       <input type = “text” name = “text2” onkeypress=”DisplayMsg(event)” size=”20”> </form> </body> </html><br /> <br /> The code in Listing 10.5 captures the value typed in the first text box and displays it in the second text box, as shown in Figure 10.5.<br /> <br /> OUTPUT FIGURE 10.5 Use the Event object to capture the actual keys pressed on the keyboard.<br /> <br /> Again keep in mind that in order to work with the Event object you need to design your code so that it is browser specific. For example, in Listing 10.5 we used the Document object with an if statement to determine which browser type was being used. If you attempt to access an Event property on a browser that does not support that property, your code will be ignored.<br /> <br /> Monitoring an Object’s Focus Another type of event that we can monitor is to determine when an object has focus. An object has focus when it is selected. For example, if you have a text box on a form, it has focus when it is selected to receive keyboard input.<br /> <br /> Events and Event Handling<br /> <br /> 345<br /> <br /> The Focus event is triggered whenever a window, frame, or form object gains focus. When the specific object is selected, it has focus. The Blur event, on the other hand, is triggered when focus is removed from an object. This event is triggered by the same objects as the Focus event. The Blur event works well for checking a form field that a user has just left and verifying the values. Listing 10.6 illustrates how to use the Focus event to determine when an object receives focus and displays a message in the status bar.<br /> <br /> INPUT<br /> <br /> LISTING 10.6<br /> <br /> Monitoring for an Event to Have Focus (focusevent.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function DisplayMsg(NumVal) { if (NumVal == 1) { status = “Type your name in the field” ; } if (NumVal == 2) { status = “Type your phone number in the field” } } //..> </script> <title>Keyboard Event</title> </head> <body> <form name=”form1”> <b>Name:</b>  <input type = “text” name = “text1” onfocus=”DisplayMsg (1)” size=”20”><p> <b>Phone:</b>  <input type = “text” name = “text2” onfocus = “DisplayMsg(2)” size=”20”></p> </form> </body> </html><br /> <br /> In Listing 10.6, the onFocus event handler is triggered whenever the field is selected, as shown in Figure 10.6. You can also monitor when an object loses focus by using the onBlur event. Keep in mind that if you use both the onFocus and onBlur event handlers for fields on a form, as you tab through fields the onBlur event fires as you leave a text box and the onFocus fires for the next text box.<br /> <br /> ANALYSIS<br /> <br /> 10<br /> <br /> 346<br /> <br /> Day 10<br /> <br /> OUTPUT FIGURE 10.6 You can use the Focus event to monitor the selection of fields on a form.<br /> <br /> Monitoring Window and Document Events JavaScript allows you to monitor events that occur relative to an actual window and the corresponding document. For example, whenever a new document loads a Load event is triggered. You can use the onLoad event handler to specify code that you want to run after the page loads. For example, the following line of code would display an alert box with a welcome message: <body onload=alert(“Welcome to my site.”)><br /> <br /> This alert box loads as soon as the page is loaded. In this case, the onLoad event handler was added to the Body tag, but it can be used to monitor the loading of any object. By the same token, you can use the Unload event to specify code to execute before unloading a particular page. If you use this code it executes as soon as the user switches to another Web page. Another useful Window event to monitor is the Resize event. This event is triggered whenever the browser window is resized. If you remember, we used this event in Listing 10.2. In that code we displayed a message each time the browser window was resized. You can also use the Move event in much the same fashion if you want to monitor when the browser window is moved.<br /> <br /> Capturing Mouse Actions The mouse is probably the most-used device for interacting with a Web page. Because of that fact, there are several different events that are triggered because of mouse actions. In fact, some mouse actions actually trigger multiple events. For example, when you click<br /> <br /> Events and Event Handling<br /> <br /> 347<br /> <br /> on a button with the mouse not only does the Click event trigger, but also the MouseDown and MouseUp events. In fact the events are actually triggered in the following order: MouseDown, MouseUp, and then Click. Of course clicking the mouse is not the only action you can perform. In fact the browser actually keeps track of the mouse cursor location and triggers a MouseMove event with each mouse movement. As the mouse moves across an object, a MouseOver event is triggered. When you move the mouse off of the object, a MouseOff event is triggered.<br /> <br /> Intercepting Mouse Clicks Probably the most commonly used event handler for the mouse is the onClick event handler. As I mentioned, this event handler indicates that the mouse has been clicked on a particular object. By adding the onClick event handler to the HTML object tag, you can customize how the Web page responds to a mouse click on a particular object. For example, the onClick event handler in the following code is triggered when the button is clicked. <input type=”button” value=”Push Button” name=”Button1” onclick=”window.alert (‘Button Pressed’);”><br /> <br /> Besides monitoring form fields for mouse clicks, you also may want to capture the Click event for a link. For example, you may want to display a message before opening a link clicked on your Web page, as illustrated in the code in Listing 10.7.<br /> <br /> INPUT<br /> <br /> LISTING 10.7<br /> <br /> Verifying Before Opening a Link (clicklink.htm)<br /> <br /> <html> <head> <script Language=”Javascript” type = “text/javascript”> <!-var newurl function CheckRequest(newurl) { if (confirm(“Do you want to visit “ + newurl + “ site.”)) { return true } else { return false } } //--> </script><br /> <br /> 10<br /> <br /> 348<br /> <br /> Day 10<br /> <br /> LISTING 10.7<br /> <br /> continued<br /> <br /> <title>Capturing Links</title> </head> <P><A href=”http://www.microsoft.com” onclick = “return CheckRequest(‘Microsoft’)” rel="nofollow">Microsoft</A></P> <P><A href=”http://www.netscape.com” onclick = “return CheckRequest(‘Netscape’)” rel="nofollow">Netscape</A></P> </body> </html><br /> <br /> As you can see in Listing 10.7, using the onClick event within a link object allows you to specify code that should execute before displaying the corresponding link. The code I used for the event handler may look slightly different from other examples that we have used. In this example, we have added the return keyword before the JavaScript function call. By doing this you can use the Confirm method to verify that the user wants to view the Web site before switching to that link. The Confirm method displays a dialog box, as shown in Figure 10.7. The method returns a value of True if the user selects the OK button; otherwise, the value of False is returned if the Cancel button is selected and the new link does not display.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 10.7 By capturing the Click event for a link, you can customize how the link displays.<br /> <br /> Of course the DblClick event can be captured using the same type of coding. If you want to capture a double click of the mouse instead of the single click, you will need to use the DblClick event handler. Keep in mind that you will want to be careful capturing both the Click and DblClick events for the same object. Of course the Click object is always going to be triggered when the first mouse click occurs, and then the DblClick event triggers after the second mouse click. Therefore, it can be difficult to distinguish whether a click or double click occurred.<br /> <br /> Events and Event Handling<br /> <br /> 349<br /> <br /> In addition to the actual clicking of the mouse, you can also capture the events of pressing and releasing the mouse using the MouseDown and MouseUp events. These two events are actually triggered before the Click event.<br /> <br /> Creating Mouse-Over Effects One of the most commonly used visual effects on current Web sites is the changing of menu items, or other items on a Web page, as the mouse cursor drags across the items. This may appear to be indicative of the use of cool graphics, but it is actually accomplished by simply using mouseover effects that are triggered by the MouseOver and MouseOut events. This process of changing images based upon the movement of the mouse is commonly referred to as mouse-over effects or hovering. When you add mouse-over effects to your Web page you are basically capturing the mouse movement across a particular image or link. If the mouse moves over the object, a MouseOver event triggers. When the event triggers you simply swap the image. When the user moves the mouse off of the object, you swap the image back to the original image. This type of event capturing and image changing gives the appearance of a much more dynamic Web page. In Listing 10.8, we create a Web page that changes the menu-button images when the mouse moves across the image. The images change again if when the mouse clicks on a button image to give the impression of a pressed button.<br /> <br /> INPUT<br /> <br /> LISTING 10.8<br /> <br /> Creating Mouse-Over Effects (mouseaction.htm)<br /> <br /> <html> <head> <title>Mouse Over Buttons</title> <script language=”JavaScript” type=”text/javascript”> <!-{ if (document.images) button1on = new Image(); button1on.src = “about-pressed.gif”; button2on = new Image(); button2on.src = “product-pressed.gif”; button3on = new Image(); button3on.src = “service-pressed.gif”; button4on = new Image(); button4on.src = “contact-pressed.gif”; button1 = new button1.src = button2 = new button2.src =<br /> <br /> Image(); “about-unselected.gif”; Image(); “product-unselected.gif”;<br /> <br /> 10<br /> <br /> 350<br /> <br /> Day 10<br /> <br /> LISTING 10.8<br /> <br /> continued<br /> <br /> button3 = new button3.src = button4 = new button4.src =<br /> <br /> Image(); “service-unselected.gif”; Image(); “contact-unselected.gif”;<br /> <br /> button1up = new button1up.src = button2up = new button2up.src = button3up = new button3up.src = button4up = new button4up.src =<br /> <br /> Image(); “about-highlighted.gif”; Image(); “product-highlighted.gif”; Image(); “service-highlighted.gif”; Image(); “contact-highlighted.gif”;<br /> <br /> } function Down(buttonname) { if (document.images) { document[buttonname].src = eval(buttonname + “on.src”); } } function Off(buttonname) { if (document.images) { document[buttonname].src = eval(buttonname + “.src”); } } function Over(buttonname) { if (document.images) { document[buttonname].src = eval(buttonname + “up.src”); } } // --> </script> </head> <body> <table width=118 border=0 cellpadding=0 cellspacing=0> <tr> <td width=1><a href=”javascript:void(0)” onmouseover=”Over(‘button1’)” onmouseout=”Off(‘button1’)” onclick=”Down(‘button1’)” rel="nofollow"> <img name=”button1” src=”about-unselected.gif” alt=”About Us” border=0 width=”119” height=”31”></a></td><br /> <br /> Events and Event Handling<br /> <br /> LISTING 10.8<br /> <br /> 351<br /> <br /> continued<br /> <br /> </tr> <tr> <td width=1><a href=”javascript:void(0)” onmouseover=”Over(‘button2’)” onmouseout=”Off(‘button2’)” onclick=”Down(‘button2’)” rel="nofollow"> <img name=”button2” src=”product-unselected.gif” alt=”Products” border=0 width=”119” height=”31”></a></td> </tr> <tr> <td width=1><a href=”javascript:void(0)” onmouseover=”Over(‘button3’)” onmouseout=”Off(‘button3’)” onclick=”Down(‘button3’)” rel="nofollow"> <img name=”button3” src=”service-unselected.gif” alt=”Services” border=0 width=”119” height=”31”></a></td> </tr> <tr> <td width=1><a href=”javascript:void(0)” onmouseover=”Over(‘button4’)” onmouseout=”Off(‘button4’)” onclick=”Down(‘button4’)” rel="nofollow"> <img name=”button4” src=”contact-unselected.gif” alt=”Contact Us” border=0 width=”119” height=”31”></a></td> </tr> </table> </body> </html><br /> <br /> As shown in Figure 10.8, the code in Listing 10.8 creates a menu that contains four button options. When the mouse cursor drags across a button image, the button appears highlighted to indicate the mouse’s position. If the mouse clicks on a button image, the button image changes to an image that gives the impression of a pressed button.<br /> <br /> OUTPUT FIGURE 10.8 Mouse-over effects allow you to create the illusion of actual buttons on your Web page.<br /> <br /> ANALYSIS You will notice that the HTML code contains a table with four links as menu items. Initially an image that represents our unselected button displays for each link.<br /> <br /> 10<br /> <br /> 352<br /> <br /> Day 10<br /> <br /> Tip<br /> <br /> You will notice that instead of assigning another page to each link, I used the statement javascript:void(0). This statement essentially disables the link so it remains on the same page. This statement works well anytime you need to disable a link that you do not want a user to visit.<br /> <br /> Also, in order for each button image to be changed when the user drags the mouse over, or clicks on the image, you need to make sure each image is named. This is done by using the name property. The OnClick, OnMouseOver, and OnMouseOff event handlers are used to specify the actions to perform when the corresponding events take place. Each event handler calls the corresponding JavaScript function by passing the name of the image that was selected. Now let’s look at the JavaScript code at the top. The first thing you want to do when creating this type of code is preload all of the graphics into Image objects. As you create the individual Image objects, you will assign the path of the graphic to the src property for each object.<br /> <br /> Note<br /> <br /> You will notice that we used an if statement before creating the Image objects. The if statement is designed to verify that the browser your code is running in actually supports the Image object. When if (document.images) returns a value of True the browser supports the Image object and your JavaScript code executes. If the statement returns a value of False, the code is ignored.<br /> <br /> After preloading all of the images, we need to create functions to handle the mouse movements. The Over() function executes whenever the MouseOver event triggers, and the OnMouseOver event handler calls the function. The Off() function executes when the MouseOut event triggers. Finally, the Down() function executes when the Click event triggers. You can expand on this basic version of the mouse-over effects as you implement it into your Web page. If you want to give the appearance of the button remaining pressed as you switch to another page, you would change the HTML code for the button to ignore the MouseOver and MouseOff events. For example, if you were displaying the Products page, the link on that page would be modified to display as follows:<br /> <br /> Events and Event Handling<br /> <br /> 353<br /> <br /> <tr> <td width=1><a href=”javascript:void(0)” onclick=”Down(‘button2’)” rel="nofollow"> <img name=”button2” src=”product-unselected.gif” alt=”Products” border=0 width=”119” height=”31”></a></td> </tr><br /> <br /> You will notice that I removed the OnMouseOver and OnMouseOut event handlers from the code. By not capturing those events for this button, the button image appears the same until you select the next link. Another thing to keep in mind, if you are switching between pages, is that it is not always necessary to add the onClick event to the HTML tag. Because as soon as the Click event occurs the link will load the new page. You can simply load the new page with the button selected image.<br /> <br /> 10 Tip<br /> <br /> There are many ways that you can make the image-over effects work on your Web page. Another common feature is to have another image modified as part of the mouse event. To change another image, you would simply identify the image to be changed as part of your JavaScript functions.<br /> <br /> Summary This chapter explored how JavaScript intercepts various browser events and allows you to execute code based upon the event being triggered. By capturing events, you are able to add a more dynamic feel to your Web site because you are performing specific actions based on the occurrence of the specific events. Each event is captured using a corresponding event handler. Typically the event handler is specified as part of the tag for the HTML object. Although we looked at the fact that some HTML objects do not have corresponding tags, you will need to create specific JavaScript code to capture those events. You can monitor keyboard, mouse, error, window, and document events and perform actions when any of those events are triggered. Probably the most commonly monitored mouse event is the Click event. This event can be captured using the onClick event handler. This event is typically used to monitor the clicking of a link or form button. Keep in mind that event handling is the basis for Dynamic HTML (DHTML), which we will discuss in detail in Chapter 11, “Dynamic HTML.”<br /> <br /> 354<br /> <br /> Day 10<br /> <br /> Workshop In the workshop today we will use the questions to review what you have learned in this chapter about working with events in JavaScript.<br /> <br /> Q&A Q. Should I validate form fields as the user types values or after the form is submitted? A. This is really a personal preference. If you have a large form with several fields, you may want to validate individual fields to be able to easily indicate to the user which fields are not valid. The downside to doing so is that the user may already intend to change form field values before submitting the form, and your messages on each field may only be an aggravation. Ideally you only validate the form fields after the form is submitted, but design your messages to properly inform the user of the fields that need to be modified. Q. When I use both the Change and Blur events they appear to fire simultaneously. Is that correct? A. The Change event always fires immediately after the Blur event. Therefore it makes more sense to only use one of the events as your event handler. Typically the Change event is more reliable and is the preferred event handler for monitoring changes to a form field. Q. How do I capture events that are triggered by objects that do not have corresponding HTML tags? A. Some objects do not have HTML object tags, such as the Document object. In order to capture these events you need to create JavaScript code that monitors these events directly. This code is created as follows: objectname.eventname = eventhandler<br /> <br /> Quiz 1. What is the best method for verifying input on a form before it is submitted? 2. What is another event to monitor to determine when a field changes? 3. How do you pass a reference to the object with the event hander JavaScript function call? 4. What is the Event object?<br /> <br /> Events and Event Handling<br /> <br /> 355<br /> <br /> Quiz Answers 1. You can use the onSubmit event handler to call a JavaScript function to verify the values in the fields on the form. For example, if you require a number within a specific range of values you can check the field to ensure that it contains the appropriate values. 2. The Change event is triggered whenever a field on a form is modified. You can use the onChange event handler to monitor the field and launch the appropriate code to validate the value typed in the field. 3. By using the this keyword as the argument for your JavaScript function call you can pass a reference to the object back to your JavaScript function. Once you have passed the object reference into your function, you can use the properties of the object to reference it. For example, the Name property can be used to reference the object name. 4. The Event object stores detailed information about the most recent event. Although the object is supported by both Netscape and Microsoft, they each support different object properties.<br /> <br /> Exercises 1. Create a Web page that uses the Focus event to display a status bar message describing what should be typed when a text box is selected. 2. Create a Web site that uses the mouse-over effects for the menus. Keep in mind you want the newly displayed pages to display the selection on the menu. Therefore, you want the selected menu option to be displayed on the new page.<br /> <br /> 10<br /> <br /> WEEK 2<br /> <br /> DAY<br /> <br /> 11<br /> <br /> Dynamic HTML Dynamic HTML, or DHTML as it is commonly called, has become widely recognized as a method for adding dynamic effects to your Web page. With DHTML you can access and modify the attributes of nearly every HTML element. This chapter will look at the elements that make up DHTML and you will learn how to combine these elements to create dynamic visual effects on your Web page. This chapter will teach you • What DHTML is • How to use Cascading Style Sheets • How to work with layers • How to change element attributes • How to move things<br /> <br /> 358<br /> <br /> Day 11<br /> <br /> What is DHTML? In the last few chapters, you have looked at different methods for creating “dynamic” effects on your Web page. For example, in Chapter 10, “Events and Events Handling,” you found out how to capture events that happened on the Web page, and how to perform actions as a result of an event. Even though we learned to do some pretty cool stuff such as creating mouse-over effects with a movement of the mouse, everything we have done has really just added a little life to an otherwise static Web page. So what options are there? After all, HTML is a just a glorified version of a printed page, right? Well actually there is another option to look at—Dynamic HTML. DHTML is a method for gaining control in order to create a dynamic Web page with the ability to modify content, layout, and design after the page has loaded. In the ideal sense, DHTML gives you total control over everything on the Web page, not only before it loads but even after it loads. Of course, keep in mind I said “ideal sense”; part of our goal in this chapter is to provide a realistic view of what can be accomplished in today’s browsers with DHTML. Okay, before you think you have to learn some other language now, let me tell you what DHTML really is. DHTML is simply a combination of HTML 4.0 (or later), Cascading Style Sheets, JavaScript, and W3C DOM to produce a “dynamic” Web page. So really the only thing that we haven’t discussed so far in this book is Cascading Style Sheets, which we will talk about in the next section. The main thing to keep in mind is that DHTML standards are only well supported by Netscape 6 or higher and Internet Explorer 5 and higher. (Other browsers support these standards too, but we are concentrating on the two main browsers.) The prior versions of these browsers, particularly the 4.0 versions, supported forms of DHTML but the support was really proprietary to each browser, making it a bigger challenge to create crossbrowser compatible code. Of course, since these browser versions are still in existence today, you need to allow for them in your coding, and we will touch on some of the issues of the earlier browser versions throughout this chapter. Because Cascading Style Sheets (CSS) play such an important role in the development of DHTML pages, we will focus a great deal of attention on CSS styles and the creation of rules. We will also focus on the various properties that you can set. Although these properties are set as CSS rules, they can also be modified through DHTML.<br /> <br /> Using Cascading Style Sheets As you are probably quite aware, styles allow you to specify how a particular block of text appears on a page. Since an HTML page is really just another way to display a<br /> <br /> Dynamic HTML<br /> <br /> 359<br /> <br /> written document, at least that was the original purpose for HTML, most of the HTML tags available allow you to modify the look of your HTML page. For example, applying any of these styles changes how your text appears: <h1>, <h2>, <h3>, <p>, <b>, <i>, <u>. Of course the list of HTML tags that affect the look of your page is really rather extensive. Therefore, your first thought might be to wonder why you would even want to bother with any other type of styles. But let’s stop and think about things for a bit. Have you ever noticed that when you apply an HTML tag to your page that the appearance can be quite different between one browser and another? Have you ever wanted to not only make the text bold, but also change the color or font? Yes, HTML with the help of individual browsers does provide some limited support for customizing styles. However, in order to create style effects that appear consistently on all browsers requires that you apply Cascading Style Sheets. As mentioned earlier, Microsoft, Netscape, and other browsers started recognizing the fact that users wanted more control over the actual presentation of their documents. Therefore, each browser started adding methods for customizing individual HTML tags. Luckily, the World Wide Web Consortium (W3C) quickly realized the need for this type of document control and developed the Cascading Style Sheet standard (CSS). CSS provides new features that you can use with a standard HTML document to control the actual style and appearance. For example, you can use code similar to the following to change the color of the Level 1 Header: <h1 style=”color: red”>Document Header 1</h1><br /> <br /> As you can hopefully begin to see, by applying CSS styles you are able to have more control over how your page appears on each browser. No longer are you bound to the individual browser’s interpretation of the particular HTML tag; you can actually customize each tag. As you can see, CSS styles can be added directly to the HTML tag by combining it with the style tag. Although this is quite handy for tags that appear only once on a page, for HTML tags that repeat, such as <p> or <b>, you also have the ability to specify the style definitions in a central location either at the top of the HTML page or in another document. We are going to discuss these three methods of style placement within this section. But first, let’s look at how to specify your styles.<br /> <br /> Defining Styles First off, in order to define any styles you must use the <style> tag within your HTML document. This tag identifies the custom styles that you want to use within the page. The style tag includes the Type attribute that indicates the type of styles being defined, in this case CSS: <style type =”text/CSS”><br /> <br /> 11<br /> <br /> 360<br /> <br /> Day 11<br /> <br /> As with most other HTML tags, the <style> tag has a closing </style> tag that marks the end of the custom styles. Between the open and closing style tags, you specify the rules to apply to each HTML tag. Creating rules will be discussed in the next section. As mentioned earlier, you can place the styles either at the top (in the head section) of your HTML page, or in a separate document. If you only want to apply this set of rules to one particular page, it is just as easy to place the styles at the top of your HTML page. To do so, your HTML code would look something like the following:<br /> <br /> INPUT<br /> <br /> <html> <head> <style type=”text/css”> h1 {color: red; text-align: center; font-family: arial} p {color: blue; font-family: times; font-size: 120%} </style> </head> <body> <h1>Heading 1</h1> <p>This is the first paragraph</p> <p>This is the second paragraph</p> </body> </html><br /> <br /> By placing these styles at the top of the page, each time a particular HTML tag is used the corresponding CSS style rule is applied to the tag. For example, the code above causes the <p> tags to create text that is blue and 20% larger than normal, as shown in Figure 11.1.<br /> <br /> OUTPUT FIGURE 11.1 By using CSS styles consistent formatting can be applied throughout an HTML page.<br /> <br /> The cool thing about the use of the styles is the fact that you don’t have to remember that you want all paragraph text to be blue or bold; the style is automatically applied to that HTML tag. Of course for tags that occur only once on the page, you can still place the<br /> <br /> Dynamic HTML<br /> <br /> 361<br /> <br /> style code within the tag, as we did earlier with the <h1> tag. In fact you can do this for all tags if you want, but that would defeat the purpose of the style sheet. <h1 style=”color: red”>Document Header 1</h1><br /> <br /> Of course placing the styles at the top of the page works well for an individual page, but what if you want to apply the styles to an entire Web site? It can be a nuisance to remember to add all your styles to the top of each page. Luckily, this process has a much simpler solution: simply create a separate style sheet that you can call from each page. Creating a separate style sheet (called an external style sheet) is similar to adding the style sheet to the top of the page. You can create the external style sheet in any text editor. There are basically two rules you must follow:<br /> <br /> NEW TERM<br /> <br /> • The style sheet file must have a .css file extension, such as styles.css. • The external style sheet must not contain any HTML tags. The file simply contains your rules (style definitions) as shown in Listing 11.1.<br /> <br /> INPUT<br /> <br /> LISTING 11.1<br /> <br /> The Contents of an External Style Sheet (styles.css)<br /> <br /> h1 {color: red; text-align: center; font-family: arial} h2 {color: green; text-align: right; font-family: arial} p {color: blue; font-family: times; font-size: 120%}<br /> <br /> You call your style sheet from any HTML page using the <link> tag. Just like with the internal style sheet, you need to place the <link> tag within the head section of your HTML tag. The <link> tag requires three different attributes: rel, type, and href, as shown in Listing 11.2, which calls the style sheet created with Listing 11.1.<br /> <br /> INPUT<br /> <br /> LISTING 11.2<br /> <br /> Calling External Style Sheet (externalstyle.htm)<br /> <br /> <html> <head> <link rel=”stylesheet” type=”text/css” href=”styles.css” /> </head> <body> <h1>This is header 1</h1> <h2>This is header 2</h2><br /> <br /> 11<br /> <br /> 362<br /> <br /> Day 11<br /> <br /> LISTING 11.2<br /> <br /> continued<br /> <br /> <p>This is the first paragraph</p> <p>This is the second paragraph</p> </body> </html><br /> <br /> When the HTML document loads it opens the associated .css file and applies those styles to the page to produce the appropriate results, as illustrated in Figure 11.2. FIGURE 11.2 An external style sheet specifies the styles to apply to the HTML document that calls it.<br /> <br /> Again, when you use the <link> tag you need to make sure you also use the appropriate attributes. These attributes, listed in Table 11.1, specify the .css file to load. TABLE 11.1<br /> <br /> Link Tag Attributes for Loading a CSS File<br /> <br /> Attribute<br /> <br /> Description<br /> <br /> href<br /> <br /> Specifies the location (URL) of the document to link to the current HTML document<br /> <br /> rel<br /> <br /> Indicates the relationship between the current HTML document and the document that is being linked by the href attribute<br /> <br /> Type<br /> <br /> Indicates the type of the file being linked by the href attribute<br /> <br /> Note<br /> <br /> You can actually have multiple CSS files linked to a single HTML document, or even an internal CSS style definition that links to an external CSS file. The confusion comes when the same HTML tag has multiple rules defined. Fortunately, there is a predefined order that the browser uses when determining which rule to apply. Rules have the following order of precedence: • Inline style (within the HTML tag) • Internal Style Sheet (within the Head section)<br /> <br /> Dynamic HTML<br /> <br /> 363<br /> <br /> • External Style Sheet (.css file) • Browser default This means that if the tag has a style definition that definition takes precedence over any other style definitions that may be available for that tag.<br /> <br /> You have total freedom to decide where you want to place your style rules. In fact, it is quite common to use a combination of an external style sheet, an internal style sheet, and inline styles within one HTML page. Of course, now that we have looked at where to place the style definitions, we need to discuss what can actually be defined, as outlined in the next section.<br /> <br /> Creating CSS Rules Each style definition within the style sheet is called a rule. Essentially a rule consists of three parts: HTML element (called a selector), and a declaration consisting of a property, and value. The rule is created by specifying the HTML element, and then specifying the property and property value in curly brackets as shown: h1 {color: red}<br /> <br /> This definition specifies that the <h1> heading level should be red. One thing to keep in mind, only the portions of the elements’ style that you specify within your CSS style sheet are modified. In other words, if the default font for the HTML element is Times Roman and you do not specify a different font, the font value of the browser is used. You can also specify the values of multiple properties for each HTML element by placing a semicolon after each declaration within the brackets: h1 {color: red; font-family: arial; text-align: center}<br /> <br /> Notice three different properties were specified for the h1 heading level. By the same token, you can also apply the same property values to multiple HTML elements using the same rule: h1, h2, h3 {color: red; font-family: arial; text-align: center}<br /> <br /> Essentially you can define properties of any HTML element within the style sheet. As you can see, the use of CSS is very valuable when creating a consistent look and feel for your Web site. You can change the alignment, background, colors, fonts, margins, padding, and list properties for each HTML element.<br /> <br /> 11<br /> <br /> 364<br /> <br /> Day 11<br /> <br /> Note<br /> <br /> Below you will find several tables with the properties you can use with your Cascading Style Sheets. Unfortunately in one chapter it is not possible to thoroughly discuss each property and its corresponding values, but these tables allow you to see the versatility you have in defining your rules.<br /> <br /> Controlling the Background Properties You can use the various background properties to control the background appearance for a particular HTML element. For example, the following code would change the background color of the HTML page to red: body {background-color: red}<br /> <br /> Besides changing the color, you can specify background images as well as the location of the background image. The properties you can set are outlined in Table 11.2. TABLE 11.2<br /> <br /> CSS Background Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> background-attachment<br /> <br /> scroll fixed<br /> <br /> Image scrolls with the rest of the page. Image remains in fixed location as page scrolls.<br /> <br /> background-color<br /> <br /> Can be a color name, RGB value, or Hex value representing the color. See Appendix B for color codes. Also, use transparent for a transparent background.<br /> <br /> background-image<br /> <br /> No background image. Can be the URL of the image to display for the background.<br /> <br /> background-position<br /> <br /> One of these values: top left, top center, top right, center left, center center, center right, bottom right, bottom center, bottom left. Also,<br /> <br /> can use x% and y% to specify horizontal and vertical positioning. Or x-pos and y-pos to specify positioning in any CSS units, such as 10px 20px. background-repeat<br /> <br /> Note<br /> <br /> repeat Repeats image vertically and horizontally. repeat-x Repeats image horizontally. repeat-y Repeats image vertically. no-repeat Does not repeat image.<br /> <br /> CSS allows you to use several different units of measurement when you specify values for various properties. The only catch is that you must specify the unit identifier after the numeric value, such as 10px represents 10 pixels. Table 11.3 lists the different units of measurement.<br /> <br /> Dynamic HTML<br /> <br /> TABLE 11.3<br /> <br /> 365<br /> <br /> CSS Units of Measurement<br /> <br /> Unit<br /> <br /> Description<br /> <br /> %<br /> <br /> Specific percentage<br /> <br /> in<br /> <br /> Inch<br /> <br /> cm<br /> <br /> Centimeter<br /> <br /> mm<br /> <br /> Millimeter<br /> <br /> em<br /> <br /> Font size of the current element<br /> <br /> ex<br /> <br /> The x-height of the font—normally about half the font size<br /> <br /> pt<br /> <br /> Point (1pt = 1/72 inch)<br /> <br /> pc<br /> <br /> Pica (1pc = 12pt)<br /> <br /> px<br /> <br /> Pixels – (1px = 1 dot on the screen)<br /> <br /> Adjusting Borders You can use the border properties to control the style, color, and width of a particular HTML element. The properties you can set are outlined in Table 11.4. TABLE 11.4<br /> <br /> CSS Border Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> border-bottom<br /> <br /> Sets properties for the bottom border by setting border-bottom color, border-bottom style, and border-bottom width.<br /> <br /> border-color<br /> <br /> Up to four values specifying top, right, bottom, and left borders. Can be a color name, RGB value, or Hex value representing the color. See Appendix B for color codes.<br /> <br /> border-left<br /> <br /> Sets properties for the left border by setting border-left color, border-left style, and border-left width.<br /> <br /> border-right<br /> <br /> Sets properties for the right border by setting border-right color, border-right style, and border-right width.<br /> <br /> border-style<br /> <br /> Up to four values specifying top, right, bottom, and left borders. Use the following values: none, hidden, dotted, dashed, solid, double, groove, ridge, inset, outset.<br /> <br /> border-top<br /> <br /> Sets properties for the top border by setting border-top color, border-top style, and border-top width.<br /> <br /> border-width<br /> <br /> Up to four values specifying the top, right, bottom, and left borders. Possible values: thin, medium, thick, or a CSS unit that indicates thickness of the border.<br /> <br /> 11<br /> <br /> 366<br /> <br /> Day 11<br /> <br /> Specify the Classification Properties You can use the classification properties to control the appearance of an HTML element, its location in respect to another HTML element, and the visibility of the HTML element. Table 11.5 outlines these properties for setting the classification of an element. TABLE 11.5<br /> <br /> CSS Classification Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> clear<br /> <br /> Indicates sides where floating elements are not allowed: left, right, both, or none<br /> <br /> cursor<br /> <br /> URL of cursor to display, or one of the following: auto, crosshair, default, pointer, move, e-resize, ne-resize, nw-resize, n-resize, se-resize, s-resize, w-resize, text, wait, help, or hand<br /> <br /> display<br /> <br /> Value indicating how element displays: none, inline, block, list-item, run-in, compact, marker, table, inline-table, table-row-group, tableheader-group, table-footer-group, table-row, table-column-group, table-cell, or table-caption<br /> <br /> float<br /> <br /> Where an image or text appears on the element: left, right, none<br /> <br /> visibility<br /> <br /> Visibility value: visible, hidden, collapse<br /> <br /> Specifying the Dimension Properties You can use the dimension properties to control the width and height of a particular image. You can also use these properties to specify the distance between lines of text. For example, the following rule sets the height and width of an <img> tag: img { height: 100px; width: 100px }<br /> <br /> Table 11.6 outlines the dimension properties you can use. TABLE 11.6<br /> <br /> CSS Dimension Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> height<br /> <br /> auto Browser uses actual image height. You can specify a numeric value in px, cm, or another CSS unit. You can also specify a percentage of the block containing the image.<br /> <br /> line-height<br /> <br /> normal<br /> <br /> width<br /> <br /> auto Browser uses actual image width. You can specify a numeric value in px, cm, or another CSS unit. You can also specify a percentage of the block containing the image.<br /> <br /> Browser sets the height. Use a number, such as 2, that is multiplied by font size to set distance. Specify a CSS unit, such as 10px. You can also specify a percentage of the current font size.<br /> <br /> Dynamic HTML<br /> <br /> 367<br /> <br /> Changing the Font Properties Probably one of the most obvious reasons for having style sheets is to have the ability to adjust the font properties for an HTML element. For example, you may want to make sure that the text is always displayed in a particular font, or that the headings are bold and underlined. Table 11.7 outlines the various font properties that you can set in the Cascading Style Sheet.<br /> <br /> Caution<br /> <br /> TABLE 11.7<br /> <br /> Keep in mind, if a browser does not support a font you specify, the browser’s default font is used. Therefore, try to use common fonts that most people have on their systems to ensure your page displays as intended.<br /> <br /> CSS Font Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> font-family<br /> <br /> List of font names to use for the HTML element. By providing multiple fonts, the browser has another font to use if the first font is not available. Separate the list with commas.<br /> <br /> font-size<br /> <br /> Specify a fixed size in pixels, such as 10px, or a percentage of the parent element. Use smaller or larger to adjust the size based on the parent element. You can also use: xx-small, x-small, small, medium, large, x-large, and xx-large.<br /> <br /> font-style<br /> <br /> normal Displays normal font. italic Displays an italic font. oblique Displays an oblique font.<br /> <br /> font-variant<br /> <br /> normal<br /> <br /> font-weight<br /> <br /> Displays normal characters. bold Displays thick characters. bolder Displays thicker characters. lighter Displays lighter characters. 100 – 900 Numeric value defining thin to thick. 400 equals normal and 700 equals bold.<br /> <br /> Displays normal font. small-caps Displays text with smallcaps.<br /> <br /> normal<br /> <br /> Setting List Properties You can customize the way your lists display using the different list style properties outlined in Table 11.8.<br /> <br /> 11<br /> <br /> 368<br /> <br /> Day 11<br /> <br /> TABLE 11.8<br /> <br /> CSS List Style Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> list-style-image<br /> <br /> Specifies the URL of the image to use as the list marker. If you do not want an image, specify the value none.<br /> <br /> list-style-position<br /> <br /> inside<br /> <br /> list-style-type<br /> <br /> One of these values to specify list marker type: none, disc, circle, square, decimal, decimal-leading-zero, lower-roman, upper-roman, lower-alpha, upper-alpha, lower-latin, upper-latin, hebrew, armenian, georgian, cjk-ideographic, hiragana, katakana, hiragana-iroha, and katakana-iroha.<br /> <br /> Indents the marker and the list. outside Keeps the marker and the text on the left.<br /> <br /> Setting the Spacing You can use the Margin properties to set the spacing around HTML elements. Whereas, the Padding properties specify the spacing between the border of the HTML element and the content, like the padding for a table. Table 11.9 shows these properties. TABLE 11.9<br /> <br /> CSS Margin and Padding Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> padding-bottom<br /> <br /> Sets the bottom padding as a CSS unit or a percentage of the element width.<br /> <br /> padding-left<br /> <br /> Sets the left padding as a CSS unit or a percentage of the element width.<br /> <br /> padding-right<br /> <br /> Sets the right padding as a CSS unit or a percentage of the element width.<br /> <br /> padding-top<br /> <br /> Sets the top padding as a CSS unit or a percentage of the element width.<br /> <br /> margin-bottom<br /> <br /> auto Browser sets bottom margin. Specify a fixed bottom margin or a percentage of total height of document.<br /> <br /> margin-left<br /> <br /> auto Browser sets left margin. Specify a fixed left margin or a percentage of a total width of document.<br /> <br /> margin-right<br /> <br /> auto Browser sets right margin. Specify a fixed right margin or a percentage of total width of document.<br /> <br /> margin-top<br /> <br /> auto Browser sets top margin. Specify a fixed top margin or a percentage of total height of document.<br /> <br /> Dynamic HTML<br /> <br /> 369<br /> <br /> Customize Text Properties You can use the various text properties to control the actual appearance of the text. Table 11.10 shows the various possibilities, such as changing the text color, adjusting the alignment of the text, and even specifying the indentation. TABLE 11.10<br /> <br /> CSS Text Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> color<br /> <br /> Can be a color name, RGB value, or Hex value representing the color. See Appendix B for color codes.<br /> <br /> letter-spacing<br /> <br /> normal Applies<br /> <br /> text-align<br /> <br /> Specifies alignment value: left, right, center, or justify.<br /> <br /> text-decoration<br /> <br /> Specifies text decoration: none, underline, overline, linethrough, or blink.<br /> <br /> text-indent<br /> <br /> Specifies a CSS unit or percentage to indent the text.<br /> <br /> text-transform<br /> <br /> Specifies how to transform text: none, capitalize, uppercase, or lowercase.<br /> <br /> white-space<br /> <br /> Ignores white space in element. pre Preserves white space in element. nowrap Text in element does not wrap.<br /> <br /> normal spacing based on font settings. Specifies a CSS unit, such as 2px, to indicate spacing between characters.<br /> <br /> normal<br /> <br /> Creating Classes CSS styles allow you to take rules a step further by creating class rules for an HTML element. A class is essentially just a subset of a particular element. For example, a common HTML element is the <a> tag used to create links within your page. You are probably used to the fact that browsers typically show the text in different colors depending upon whether the user has visited that link recently. However, with CSS you can actually specify the colors and any other attributes that you want applied to each stage of the <a> tag. For example, the following code assigns a different color to the active (selected), visited, link (unvisited), and hover (with mouse) states of the link. a:active {color:blue} a:visted {color:red} a:link {color:green} a:hover {color:yellow}<br /> <br /> By applying these classes to your HTML document the links change colors based on their current state.<br /> <br /> 11<br /> <br /> 370<br /> <br /> Day 11<br /> <br /> You can create your own classes for HTML elements in much the same fashion. For example, you may want to apply different formatting to the first paragraph of each page. To do this you would first create the paragraph style in your Cascading Style Sheet and then you would call that style as part of the <p> tag, as illustrated in Listing 11.3.<br /> <br /> INPUT<br /> <br /> LISTING 11.3<br /> <br /> Using CSS Classes (subclass.htm)<br /> <br /> <html> <head> <style type=”text/css”> h1 {color: red; text-align:center} p {font-family: arial} p.paragraph1 {color:blue; font-weight:bold} </style> </head> <body> <h1>Creating Custom Classes</h1> <p class=”paragraph1”>This is the first paragraph</p> <p>This is the second paragraph</p> </body> </html><br /> <br /> Note that to create the class rule, you type the name of the HTML element followed by a period and the name of the class. Within your HTML document, you specify the class rule by typing the HTML tag followed by class= and the name of the class. As you will notice in Figure 11.3, the class element receives both the styles for the <p> element and <p class=”paragraph1”>.<br /> <br /> Tip<br /> <br /> You can also define a class rule that can be assigned to any element. This is useful when you have styles that you want to apply to multiple elements. To do so, simply type a period followed by the class name. For example, boldtext {font-weight:bold} creates a class that can now be applied to multiple HTML elements. You call this class using the same method as above, but by typing class=”classname”.<br /> <br /> CSS does provide a couple of built-in psuedo-classes that you can use with the <p> tag. The first-line psuedo-class allows you to specify a rule for the first line of a paragraph. You can use the first-letter psuedo-class to define a rule for the first letter of the paragraph. To create the rules for these psuedo-classes you use the same approach as above<br /> <br /> Dynamic HTML<br /> <br /> 371<br /> <br /> by typing the psuedo-class rule in the style sheet section, only you place a colon, not a period, between the element and the psuedo-class. P:first-line {color:blue} P:first-letter{font-size:150%}<br /> <br /> OUTPUT FIGURE 11.3 By creating CSS class rules you can create custom definitions that you can apply to specific HTML elements.<br /> <br /> Unlike the custom classes that you create, when your HTML document loads, the rules defined for these styles are automatically applied to each <p> paragraph.<br /> <br /> Working with Layers Netscape originally added the concept of “layers” to Netscape 4.0 to provide the ability to define a portion of the page that could be positioned, moved, or even hidden on the screen using DHTML. In order to accomplish this, each layer’s elements where placed within the <layer> tags. Although this was a great concept, it was proprietary, making it only supported by Netscape Navigator 4+ browsers, although they are not supported by Netscape Navigator 6. Fortunately the CSS specification also included a positioning specification that could be used with all HTML elements. To replace the proprietary <layer> element, you can use the <div> element that is supported by all browsers that support HTML 3.0 or higher. Just like the layering concept, the <div> element defines a group or section of the page. By combining the CSS positioning properties with the <div> element you can create the layering effects first introduced by Netscape. There are several different CSS positioning properties that you can use to specify the position and shape of the HTML element. The properties are outlined in Table 11.11.<br /> <br /> 11<br /> <br /> 372<br /> <br /> Day 11<br /> <br /> TABLE 11.11<br /> <br /> CSS Positioning Properties<br /> <br /> Property<br /> <br /> Values<br /> <br /> bottom<br /> <br /> auto<br /> <br /> clip<br /> <br /> Browser sets the clipped size. Specifies the size as rect (top, right, bottom, left). For example, rect(10px, 5px, 10px, 4px).<br /> <br /> left<br /> <br /> auto<br /> <br /> Browser calculates bottom position of element. Specifies a CSS unit, such as 100px, or a percentage, for position from the bottom of a parent block.<br /> <br /> auto<br /> <br /> Browser calculates left position of element. Specifies a CSS unit, such as 100px, or a percentage, for position from the left of a parent block.<br /> <br /> overflow<br /> <br /> Displays entire element outside specified size. Content clipped to fit. scroll Content clipped to fit and scrollbar adds to view rest of element. auto Browser displays scrollbar if content is clipped. auto<br /> <br /> hidden<br /> <br /> position<br /> <br /> static<br /> <br /> Items are laid out in normal HTML fashion and cannot be<br /> <br /> moved. Item is positioned based upon specified coordinates. Item is positioned based upon an offset from the static location where HTML would have placed the item.<br /> <br /> absolute relative<br /> <br /> Browser calculates right position of element. Specifies a CSS unit, such as 100px, or a percentage, for position from the right of a parent block.<br /> <br /> right<br /> <br /> auto<br /> <br /> top<br /> <br /> auto<br /> <br /> vertical-align<br /> <br /> Value specifying vertical alignment: baseline, sub, super, top, texttop, middle, bottom, text-bottom. You can also specify a CSS unit, such as 100px, or percentage, indicating vertical alignment.<br /> <br /> Browser calculates top position of element. Specifies a CSS unit, such as 100px, or a percentage, for position from the top of a parent block.<br /> <br /> Probably the most used positioning property is the Position property. By using the absolute and relative positioning values you can specify exactly where on the page you want the particular layer to be placed. For example, the code in Listing 11.4 illustrates how to define the absolute positioning for a “layer.”<br /> <br /> INPUT<br /> <br /> LISTING 11.4<br /> <br /> Specifying the Positioning of a Layer (PositionLayer.htm)<br /> <br /> <html> <head> <style type=”text/css”><br /> <br /> Dynamic HTML<br /> <br /> LISTING 11.4<br /> <br /> 373<br /> <br /> continued<br /> <br /> div.layer1 {position:absolute; left:75px; top:75px; color:red} div.layer2 {position:absolute; right:75px; bottom:75px; font-family: arial; color:blue; font-weight:bold} </style> </head> <body> <div class=”layer1”> <h1>Positioning Layers</h1> <p>This is the first layer1 paragraph</p> <p>This is the second layer1 paragraph</p> </div> <div class=”layer2”> <p>This is the first layer2 paragraph</p> <p>This is the second layer2 paragraph</p> </div> </body> </html><br /> <br /> Notice the fact that not only did we use the positioning properties with the <div> element but also some of the other CSS properties for specifying the appearance of the text within each layer. By using absolute positioning, each layer is placed on the screen at the specified position. HTML does not care how the positioning affects another layer. In fact, depending upon how the browser window is sized, the layers may even overlay, as shown in Figure 11.4.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 11.4 Use the <div> HTML element to create layers on your page.<br /> <br /> 11<br /> <br /> 374<br /> <br /> Day 11<br /> <br /> Note<br /> <br /> Keep in mind, HTML allows you to nest <div> layers. This means you can place one or more layers inside another layer. When you do this, the rules of the parent layer are applied to all child layers. But if any of the properties are changed within the child layer rules, those new property values are applied to the child only.<br /> <br /> Now that you have seen how to create layering effects within your HTML document, let’s look at how to change those properties within your JavaScript code to create a more “dynamic” page.<br /> <br /> Changing Attributes of an HTML Element Now that we have discussed CSS and how layers can be used to apply CSS rules to multiple elements, we are ready to look at the process of changing the value of a property. In order to do this effectively, we need to use the W3C DOM that we talked about in Chapter 8, “The Browser Issue.” If you remember our discussion in Chapter 8, we looked at how the DOM specifies the elements of the HTML document in a tree-like structure with the Document object being the base of the tree and all other elements of the page are child nodes of the Document object node. We also discussed that the easiest method for accessing a particular element is to use the getElementById() method. This method allows you to access any HTML element based on the ID attribute. Of course accessing a property of an HTML element is only half of what you can accomplish with the W3C DOM; you can also change the property in much the same fashion. In fact, you may not realize that you can even change text after it has been written to the screen, as illustrated in Listing 11.5 where the text for a paragraph changes when the button is clicked.<br /> <br /> INPUT<br /> <br /> LISTING 11.5<br /> <br /> Changing Window Text (ChangeText.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function ChangeText() { var curtextval = document.getElementById(“ptext”); curtextval.innerHTML = “New Paragraph 1” }<br /> <br /> Dynamic HTML<br /> <br /> LISTING 11.5<br /> <br /> 375<br /> <br /> continued<br /> <br /> //..> </script> </head> <body> <form> <p id=”ptext”>This is the first paragraph</p> <p>This is the second paragraph</p> <input type=”button” value=”Click to Change Text” onclick=”ChangeText()”></p> </form> </body> </html><br /> <br /> As you can see, it requires very little coding to create dynamic effects that change the context of the window after it has been loaded. As you can see in Figure 11.5, the text in the first paragraph changes as soon as the button is clicked. FIGURE 11.5 With the use of the DOM, you can change the characteristics of HTML elements at any time.<br /> <br /> You may have occasions when the attribute that you want to add to an HTML tag does not currently exist for that tag. If that is the case, you will need to use the setAttribute() method. This method requires two parameters: the name of the attribute to add and the value of the attribute. A good use of this method would be to specify that a link open in another window by adding the target attribute. selectedlink.setAttribute(“target”, “_blank”)<br /> <br /> On the other hand, you may want to remove an attribute from an element. This can be accomplished by using the removeAttribute() method. This method simply requires the name of the attribute to be removed. selectedlink.removeAttribute(“target”)<br /> <br /> Listing 11.6 illustrates how to combine changing attribute values by setting and removing attributes.<br /> <br /> 11<br /> <br /> 376<br /> <br /> Day 11<br /> <br /> INPUT<br /> <br /> LISTING 11.6<br /> <br /> Changing URL Text (ChangeAttribute.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function ChangeLink() { var newlink = document.getElementById(“urltext”); var curlink = document.getElementById(“oldurl”); curlink.href = (“http://” + newlink.value); curlink.innerHTML = newlink.value; } function NewWindow(prefval) { var curlink = document.getElementById(“oldurl”); if (prefval == 1) { curlink.setAttribute(“target”, “_blank”); } else if (prefval == 2) { curlink.removeAttribute(“target”); } } //..> </script> </head> <body> <form> <a id=”oldurl” href=”http://www.microsoft.com” rel="nofollow">www.microsoft.com</a> <p><input type=”radio” value=”V1” name=”R1” onClick=”NewWindow(1)”>Open in New Window</p> <p><input type=”radio” name=”R1” checked value=”V2” onClick=”NewWindow(2)”>Open in Same Window</p> <input type=”text” id=”urltext” size=”20”>  <input type=”button” value=”Change Link” name=”B3” onClick=”ChangeLink()”></p> </form> </body> </html><br /> <br /> As you can see in Figure 11.6, the code in Listing 11.6 allows you to change not only the URL that displays when the user clicks the link, but also the text describing the URL to match the new URL name. The setAttribute() and removeAttribute() methods are used to specify whether or not the link opens in a new window.<br /> <br /> Dynamic HTML<br /> <br /> 377<br /> <br /> OUTPUT FIGURE 11.6 Changing the attributes of HTML elements allows you to create dynamic effects on the page based upon user selections.<br /> <br /> Keep in mind, you can use any of the property values we discussed with Cascading Style Sheets to change the attributes of an HTML element. Of course changing the attributes of HTML elements is quite a cool effect, but there are even more effects that can be created using DHTML, as outlined in the following section.<br /> <br /> Moving Things Now that we have established a basis for DHTML the list is quite long of the effects you can create on your HTML page. Effects such as animating images, scrolling text, keeping portions of your page visible when the user scrolls, and even creating drop-down menus can all be accomplished through DHTML. In the remaining portion of this chapter, let’s look at how to go about moving things dynamically on the screen. One of the most common reasons people decide to use DHTML is to add some type of animation to their pages. Using DHTML you can animate an element of the page whether it is text or an image. For example, in Listing 11.7 we scroll the text within the <div> layer across the screen.<br /> <br /> INPUT<br /> <br /> LISTING 11.7<br /> <br /> Animating Text (animatetext.htm)<br /> <br /> <html> <head> <style> div { position:relative; color:red} </style> <title>Animating Text</title> <script language=”JavaScript” type = “text/javascript”> var pos1=0; function next() { pos1 += 5;<br /> <br /> 11<br /> <br /> 378<br /> <br /> Day 11<br /> <br /> LISTING 11.7<br /> <br /> continued<br /> <br /> if (pos1 > 640) pos1 = 0; document.getElementById(“movetext”).style.left = pos1; window.setTimeout(“next();”,30); } </script> </head> <body onLoad=”next();”> <div id=”movetext”> <b>DHTML provides the ability to animate any <br> HTML elements on your browser window.</b> </div> </body> </html><br /> <br /> This script moves the text across the screen from left to right, as shown in Figure 11.7. When the position of the text is beyond 640, the position changes back to 0 so that it appears to wrap around the screen. Keep in mind, 640 is used as the maximum position value because the browser window was sized to 640x480. To run this script on a larger browser, you can adjust that value.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 11.7 Creating animations of images and text is one of the popular uses of DHTML.<br /> <br /> You can reverse the scroll direction from right to left by changing the script as follows: <script language=”JavaScript” type = “text/javascript”> var pos1=640; function next() { pos1 -= 5; if (pos1 < 0) pos1 = 640; document.getElementById(“movetext”).style.left = pos1; window.setTimeout(“next();”,30); } </script><br /> <br /> Notice that now the position is checked to see if it is less than zero; if so, set the position of the text back to the right side of the browser.<br /> <br /> Dynamic HTML<br /> <br /> 379<br /> <br /> The types of animations available with DHTML are limitless when you use the CSS Positioning properties covered in Table 11.11. Another very common use, and probably more useful, of DHTML is to create menus. With DHTML there are many different types of menu systems you can create. In Chapter 10, “Events and Events Handling,” we looked at creating a mouse-over menu where the buttons changed based on the location of the mouse cursor. If you remember, this menu-creation option required the use of three different graphic buttons for each menu option to give the appearance of the button changing. In Listing 11.8 we create a similar menu, but this time it is created with DHTML. By using DHTML we are able to change the contents of the table cells based on the mouse location.<br /> <br /> INPUT<br /> <br /> LISTING 11.8<br /> <br /> Dynamic Menus (dynamicmenu.htm)<br /> <br /> <html> <head> <style type=”text/css”> td.menu {font-family:Arial; font-weight:bold; background-color:blue; cursor:hand} a{ text-decoration:none; color:black; } </style> <script language=”javascript” type = “text/javascript”> <!-function selectlink(sellink,linkdesc){ sellink.style.background=’yellow’; if (document.getElementById) document.getElementById(“selectdesc”).innerHTML=linkdesc; else selectdesc.innerHTML=html; } function leavelink(sellink){ sellink.style.background=’blue’; if (document.getElementById) document.getElementById(“selectdesc”).innerHTML=’ ’; else selectdesc.innerHTML=’ ’; } //--> </script> </head> <body> <table bgcolor=”black” border=”1” bordercolor=”white” cellpadding=”2”<br /> <br /> 11<br /> <br /> 380<br /> <br /> Day 11<br /> <br /> LISTING 11.8<br /> <br /> continued<br /> <br /> cellspacing=”0”> <tr> <td class=”menu” bordercolor=”black” id=”choice1” onmouseover=”selectlink(this,’Developers of Internet Explorer’)” onmouseout=”leavelink(this)””> <a href=”http://www.microsoft.com” rel="nofollow">Microsoft</a></td></tr> <td class=”menu” bordercolor=”black” id=”choice2” onmouseover=”selectlink(this,’Developers of Netscape Navigator’)” onmouseout=”leavelink(this)”> <a href=”http://www.netscape.com” rel="nofollow">Netscape</a></td></tr> <td class=”menu” bordercolor=”black” id=”choice3” onmouseover=”selectlink(this,’Responsible for the W3C DOM’)” onmouseout=”leavelink(this)”> <a href=”http://www.w3.org/” rel="nofollow">W3C</a></td></tr> <td class=”menu” bordercolor=”black” id=”choice4” onmouseover=”selectlink(this,’Publisher of Computer Books’)” onmouseout=”leavelink(this)”> <a href=”http://www.samspublishing.com” rel="nofollow">Sams Publishing</a></td></tr> <tr> <td bordercolor=”black” bgcolor=”white” height=”18”> <font id=”selectdesc” size=”2”></font></td></tr> </table> </body> </html><br /> <br /> If you run the code in Listing 11.8, you will see that we added an additional feature to this menu. When the user drags the cursor across a menu option, a description of the menu option displays in the last table cell, as shown in Figure 11.8.<br /> <br /> OUTPUT FIGURE 11.8 You can create some dynamic menus using DHTML.<br /> <br /> Summary In this chapter, you first learned that DHTML is actually comprised of four different components: CSS, HTML 4.0 (or later), JavaScript, and the W3C DOM. Cascading Style<br /> <br /> Dynamic HTML<br /> <br /> 381<br /> <br /> Sheets (CSS) provide a means for creating style rules that can be applied to an entire document, or for that matter an entire site. You can create rules for any HTML element. You can also create rules for sub-class elements, which are simply a sub-class of a particular element. CSS provides several different types of properties that you can apply to your HTML elements. For example, to set a particular element to be underlined you set the text-decoration property value to underline. You can place CSS rules in three different locations. They can be part of the HTML tag, listed at the top of the HTML page in the Head section, or exist in an external style sheet. Netscape introduced the concept of layers as an addition to DHTML to provide a means for applying dynamic effects to portions of a page. Unfortunately layers are specific only to Netscape 4+, but the same effect can be accomplished on a cross-browser basis using the <div> element. Finally, we looked at how to combine the elements of DHTML to create dynamic effects on your page such as changing the values of static text on the page, animating elements, and even creation of dynamic menus.<br /> <br /> Workshop In the workshop today we will use the Q&A to look at other issues you may face with DHTML, and we’ll use the questions and exercises to review what you have learned in this chapter about working with DHTML.<br /> <br /> Q&A Q. How do I avoid conflicts between styles within the document and those in an external style sheet? A. Keep in mind, the styles applied as part of the HTML tag will always supercede any other styles related to that same element. The best practice is to create your general styles and store them in the external style sheet to be applied to all pages. Then if you have special situations where you want to change a style, make that change at the tag level. Q. Why should I bother to create Cascading Style Sheets? A. By creating styles you can specify how a particular HTML element should appear on the page. For example, you may want your heading levels to be a specific font style and size. You create specific rules that define the styles to apply to each element.<br /> <br /> 11<br /> <br /> 382<br /> <br /> Day 11<br /> <br /> Q. How do I apply the layer effect of Netscape 4+ in cross-browser code? A. Layers were first introduced by Netscape as a means to group portions of the page together. You can apply layering effects to all browsers using the <div> tag. With the use of layering you can set the location of each layer independently.<br /> <br /> Quiz 1. What advantage is gained by creating a separate CSS file for your style rules? 2. What is the precedence order for applying CSS styles? 3. How do you change the text in a paragraph using DHTML?<br /> <br /> Quiz Answers 1. You actually have the ability to place your CSS rules in three different locations. You can add the style information directly to the HTML tag by using the style attribute as part of the tag. For styles that appear frequently within the page you can create a CSS-style page within the Head section of the HTML page. If you want the styles applied to multiple pages on your site, you can create a .css file containing the rules that can be called by each page. 2. With CSS you have the potential of having multiple styles for the same HTML element. If rules exist for the same property of the HTML element, the rules are applied using the following order: inline style (with HTML tag), internal style sheet, external style sheet, and finally the browser default. In other words, if the rule exists in the inline style, that is the rule that is applied for that property— regardless of what the rule in the internal style sheet states. 3. In order to modify the value of a specific HTML element you must access the element by its ID attribute. Therefore, for each HTML element that you want to be dynamic, you must add an ID attribute to the corresponding HTML tag. You can access the HTML element using the getElementById method.<br /> <br /> Exercises 1. Create a Web page that scrolls text or images vertically on the page. 2. Create a Web site that has a dynamic menu that displays across the top of the page. Try adding a description of the menu link to the status bar.<br /> <br /> WEEK 2<br /> <br /> DAY<br /> <br /> 12<br /> <br /> Windows and Frames The Window object is the top-most, or main, object in each browser. Consequently, all other objects are child objects of the Window object. In this chapter, we will look at the Window object and how it compares to the Frame object. We will see that frames are nothing more than a method for displaying multiple windows within one browser window; however, each frame is independent. We will also look at several aspects of the Window object, and how they can be accessed from JavaScript. These elements are the Location object, the History object, and the Window object methods for displaying popups. We will also look at how to make changes to the status line of your window. This chapter will teach you • What windows and frames are • How to determine window location • How to work with the window history • How to work with the status line • How to use the Screen object<br /> <br /> 384<br /> <br /> Day 12<br /> <br /> • How to work with frames • How to work with popups<br /> <br /> What Are Windows and Frames? You are used to hearing the word “window.” After all, the word appears in the name of every Microsoft operating system. If you remember our discussion of objects in Chapter 4, “JavaScript Is Object-Based,” you know that with JavaScript the Window object represents the browser window. In other words, it is the main object and all other objects are children of the Window object. The Window object contains three different objects—the Document object (discussed in Chapter 11, “Dynamic HTML,” which contains all other page elements), the Location object, and the History object (each of which will be discussed in this chapter). A frame, on the other hand, is very similar to a window. In fact, it is essentially a separate window in your browser window. By using frames you can have multiple pages open simultaneously within your browser window. Each frame functions independently of the other frames on the page. Therefore, because each frame contains all the properties of a window, each frame is considered to be a Window object. Figure 12.1 illustrates how two frames can display within one browser window. FIGURE 12.1 Frames are often used to display multiple documents on one page.<br /> <br /> We will delve into frames in more detail later in this chapter. First let’s look at windows in a little more detail.<br /> <br /> Windows and Frames<br /> <br /> 385<br /> <br /> Because JavaScript treats each frame as a separate window, you will see later that dealing with multiple windows and frames can get a little confusing. In order to keep track of each window and frame, you need to have a method of differentiating between them. The best method for doing so is to assign a window to a variable. That way, you can reference the appropriate window or frame based on its variable name. first_window = window<br /> <br /> Once you have assigned the window to the variable, you can reference the window from any location. By doing so you can make modifications to its properties.<br /> <br /> Window Properties and Methods Chapter 4 discussed the method of referencing properties and methods for an object. Just like all other objects, the Window object has its own properties and methods that you can use. Chapter 4 also discussed the fact that the standard notation for referencing an object’s property or method is to specify the object name, a period, and the property name or method, as shown below: window.close()<br /> <br /> With JavaScript, the Window object is the default object. This means that if JavaScript encounters a property of a method that does not have an object specified, it automatically assumes that the property or method belongs to the Window object. Therefore, you can also specify a property or method without the object reference, shown as follows: alert(“Hi”)<br /> <br /> Also mentioned earlier was the fact that you could use a variable to reference a specific Window object, which is ideal when referencing another window or frame (as we will talk about later). However, JavaScript also provides a handy self property that you can use to ensure that the property and method calls are associated with the current window. To use this property, you simply type it, followed by the property or method you want to use, as shown below: self.close()<br /> <br /> The list of properties and methods that are associated with the Window object is quite extensive. Of course, as with other JavaScript objects, some methods and properties are browser-dependent and should probably be avoided when creating cross-browser code. Probably the most common events you will use with the Window object are open() and These methods allow you to open (create) new windows and close open windows. close().<br /> <br /> 12<br /> <br /> 386<br /> <br /> Day 12<br /> <br /> Opening a Window If you have done much work with HTML you know that opening a new window is a fairly simple task that you can accomplish by simply setting the target attribute of the <a href rel="nofollow"> HTML tag. <a href=”http://www.samspublishing.com” target=”_blank” rel="nofollow">Sams Publishing</a><br /> <br /> With the above statement, the specified Web site is opened in a new window because we specified a value of _blank for the target attribute. In fact, with the target attribute you actually have the option of specifying which window receives the new document, as outlined in Table 12.1. TABLE 12.1<br /> <br /> Target Attributes for Opening a Window<br /> <br /> Attribute<br /> <br /> Description<br /> <br /> _blank<br /> <br /> Opens the specified Web page within a new window<br /> <br /> _parent<br /> <br /> Opens the specified Web page within the parent frame<br /> <br /> _self<br /> <br /> Opens the specified Web page within the current window or frame<br /> <br /> _top<br /> <br /> Opens the specified Web page within the current window, eliminating the existing frames<br /> <br /> Another thing to be aware of is the fact you can also use the target attribute to not only load the Web page in a new window but also to assign a name to the window. By doing so, you are able to assign new Web pages to that same window using the same value for the target attribute. See example shown below: <a href=”http://www.samspublishing.com” target=”sams” rel="nofollow">Sams Publishing</a><br /> <br /> Of course I am sure by now you are thinking this is all good HTML information, but what does it really have to do with JavaScript? I gave you these HTML basics and a foundation for what you can accomplish with the open() method of the Window object. Essentially you can use the open() method within your JavaScript code to open an HTML page in much the same fashion as using the <a href rel="nofollow"> tag. Actually, in practice, you have even more capability with this method than you do in HTML. In order to use the open() method, there are basically two values you must specify: the URL of the file to open and the name of the window to open. For example, to open the Sams Publishing site, you would type the following: window.open(“http://www.samspublishing.com”, “Sams”);<br /> <br /> Windows and Frames<br /> <br /> 387<br /> <br /> When you call the open() method it actually returns a reference to the new window, so if you assign the open() method return value in a variable, you can reference the window later using the variable. winvar = window.open(“http://www.samspublishing.com”, “Sams”);<br /> <br /> Besides specifying the window name, you can also specify the size and position of the browser window when it opens. By default, the new window opens as the same size as the default browser window. You can use the attributes in Table 12.2 to change the size and location of the new window. TABLE 12.2<br /> <br /> Positioning Attributes for the open() Method<br /> <br /> Attribute<br /> <br /> Description<br /> <br /> height<br /> <br /> Indicates the height of the new window in pixels<br /> <br /> width<br /> <br /> Indicates the width of the new window in pixels<br /> <br /> left<br /> <br /> The number of pixels from the left edge of the screen to the left edge of the window<br /> <br /> top<br /> <br /> The number of pixels from the top edge of the screen to the top edge of the window<br /> <br /> For example, you can create a new window that is 300 pixels wide and 300 pixels high by typing the following: winvar = window.open(“http://www.samspublishing.com”, “Sams”, “height = 300, width = 300”);<br /> <br /> You can also specify information about the appearance of the window, such as NEW TERM whether there are toolbars, scrollbars, and so forth displayed for the new window. These window features are commonly referred to as the window chrome. Table 12.3 lists the window chrome features that you can specify for the window. TABLE 12.3<br /> <br /> Window Chrome Features with the open() Method<br /> <br /> Feature<br /> <br /> Description<br /> <br /> directories<br /> <br /> Displays the Links bar in Internet Explorer, the Personal toolbar in Netscape 4.0 or later, or the Directory buttons in Netscape 2 and 3<br /> <br /> location<br /> <br /> Displays the Address box in Internet Explorer, or the Location box in Netscape (This feature only displays if the toolbar is also displayed.)<br /> <br /> menubar<br /> <br /> Displays the Menu bar<br /> <br /> resizable<br /> <br /> Indicates that the user can resize the window<br /> <br /> 12<br /> <br /> 388<br /> <br /> Day 12<br /> <br /> TABLE 12.3<br /> <br /> continued<br /> <br /> Feature<br /> <br /> Description<br /> <br /> scrollbars<br /> <br /> Displays vertical and horizontal scrollbars, if needed<br /> <br /> status<br /> <br /> Displays the status bar<br /> <br /> toolbar<br /> <br /> Displays the toolbar<br /> <br /> By using these window chrome features you can easily customize how a particular window displays. For example, if you pop up a page in a separate window, you may want to eliminate the navigational ability within that window by removing the Address box. The code in Listing 12.1 illustrates how to customize the appearance of the window by allowing the user to select the desired window chrome features.<br /> <br /> INPUT<br /> <br /> LISTING 12.1<br /> <br /> Opening a Custom Window (openwindow.htm)<br /> <br /> <html> <head> <title>Open Window</title> <script language=”JavaScript” type =” text/javascript”> <!-function ChangeLink(currentform) { var window_feature = “”; for (var cb=0; cb<currentform.length - 1; cb++) { if (currentform[cb].checked) { currentform[cb].value = 1; } else { currentform[cb].value = 0; } window_feature = (window_feature + “,” + currentform[cb].name + “=” ➥+ currentform[cb].value); } window.open(“http://www.samspublishing.com”, “_blank”, window_feature); } //..> </script> </head> <body> <form name=”form1”> <input type=”checkbox” name=”directories” value=”0”>Directories Option</p> <p><input type=”checkbox” name=”location” value=”0”>Location Option</p><br /> <br /> Windows and Frames<br /> <br /> LISTING 12.1<br /> <br /> 389<br /> <br /> continued<br /> <br /> <p><input type=”checkbox” name=”menubar” value=”0”>Menu Bar</p> <p><input type=”checkbox” name=”resized” value=”0”>Allow Window to be Resized</p> <p><input type=”checkbox” name=”scrollbars” value=”0”>Scrollbars</p> <p><input type=”checkbox” name=”status” value=”0”>Status Bar</p> <p><input type=”checkbox” name=”toolbar” value=”0”>Toolbar</p> <p><input type=”button” value=”Create Window” name=”CreateWin” onClick=”ChangeLink(this.form)”></p> </form> </body> </html><br /> <br /> If you notice we use a for loop to cycle through the checkboxes on the form and to build the window_feature string, indicating the window options to display. Since there are actually eight options on the form, seven checkboxes and one button, we subtracted one from the length property value to ignore that form field. The user selects the checkboxes corresponding to the desired window properties, as shown in Figure 12.2. When the button is clicked the new window opens as specified.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 12.2 Use the Window Chrome features to customize the look of the window you want to open.<br /> <br /> Closing a Window Of course if you can open a window, you also need the ability to close a window. You can close a window from JavaScript using the close() method. To close the current window, you would use the following code: window.close()<br /> <br /> 12<br /> <br /> 390<br /> <br /> Day 12<br /> <br /> When your command executes, the current browser window will close. Typically though, you are going to want to close yet another window with this command. In order to do this, the window you want to close needs to be referenced. The easiest method is to assign the statement that opens the window to a variable. As mentioned in the previous section, this gives the variable a reference to the specified window thereby allowing you to reference the window at any point to close it, as shown below: ourwindow = window.open(“http://www.microsoft.com/”); ourwindow.close();<br /> <br /> You should be aware that JavaScript has a closed property for each Window object. When a window closes, the closed property is set to True. If the window is still open, the closed property is set to False. You can use this property to determine whether or not you need to close a window. You should also be aware that the property returns a null value if the window was never opened.<br /> <br /> Determining Window Location In terms of JavaScript and the Window object, location simply specifies the URL, or location of the Web page that displays in the window. There are two methods to work with the window location: • Use the Location property of the Window object. For example, to set the location of a window called mywindow you type the following: mywindow.location = “http://www.netscape.com”;<br /> <br /> • Use the Location object to set or return the URL information using the Location object properties discussed in Table 12.4. For example, you would type the following to set the window location using the Location object: mywindow.location.href = “http://www.netscape.com”;<br /> <br /> Note<br /> <br /> It may seem a little odd that there is a Location property for the Window object and a Location object. This is not at all unusual. In fact the Location property of the Window object actually loads the Location object with the specified URL. Essentually, each child object of an object is actually a property of the same object.<br /> <br /> As we just specified, the Location object represents the actual URL for the page. Although the Location object only contains a URL, it does have several properties you can use to examine parts of the URL, as outlined in Table 12.4.<br /> <br /> Windows and Frames<br /> <br /> TABLE 12.4<br /> <br /> 391<br /> <br /> Location Object Properties<br /> <br /> Property<br /> <br /> Description<br /> <br /> hash<br /> <br /> The name of the anchor specified in the URL address. The anchor value is the portion of the address that follows the hash sign (#). Anchors are typically set to jump to a specific location on a page, such as #top.<br /> <br /> host<br /> <br /> The host name and the port used to communicate with the server<br /> <br /> hostname<br /> <br /> The host name of the server<br /> <br /> href<br /> <br /> The entire URL address<br /> <br /> pathname<br /> <br /> The directory path and filename of the document<br /> <br /> port<br /> <br /> The port used by the browser to communicate with the server<br /> <br /> protocol<br /> <br /> The protocol used for communication between the browser and the server (Common protocols are http and ftp.)<br /> <br /> search<br /> <br /> The portion of the URL that contains the search string. This is the value that follows a ? in the URL<br /> <br /> So, you can change the URL displayed by an open window by either using the window.location property or the Location.href property. Actually, when you do use Window.location to change the URL, JavaScript is actually just changing the href property of the corresponding Location object. For example, the code in Listing 12.2 illustrates how to change the URL displayed in one frame to the value specified by the user in the other frame. Keep in mind working with frames will be discussed in more detail later in this chapter.<br /> <br /> INPUT<br /> <br /> 12 LISTING 12.2<br /> <br /> Changing the Window URL (changelocation.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function changeLocation() { parent.frames[1].location = document.form1.NewURL.value; } //..> </script> </head> <body> <title>Specify URL</title> </head> <body><br /> <br /> 392<br /> <br /> Day 12<br /> <br /> LISTING 12.2<br /> <br /> continued<br /> <br /> <form name=”form1”> <input type=”text” name=”NewURL” size=”50”> <input type=”button” value=”Change URL” name=”URLButton” onclick = “changeLocation()”></p> </form> </body> </html><br /> <br /> You will notice that we use the href property of the Location object to set the URL for the second frame, as shown in Figure 12.3. Keep in mind, the href property is the default property for the Location object. Therefore, it is not necessary to specify the property name. In fact, if you omit it, JavaScript automatically applies the changes to that property.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 12.3 The Location object maintains the URL information for the window.<br /> <br /> The Location object also has three methods that you can use: the assign() method, the reload() method, and the replace() method. Each method is fairly easy to use. Remember there are two methods that you can use to assign a URL. You can either use the Location property of the Window object or set the href property of the Location object. Whichever method you decide to use, JavaScript uses the assign() method to assign the new URL to the browser window. You can also use this method, if desired, but it is not necessary. To use it, you simply specify the new URL, as follows: location.assign(“http://www.netscape.com”);<br /> <br /> Windows and Frames<br /> <br /> 393<br /> <br /> You can use the reload() method to reload the current page in the browser. This is basically the same as selecting the Refresh or Reload option within the browser. The only difference is that you can specify whether the page reloads from the browser cache or from the server using the optional Source attribute, as follows: location.reload<br /> <br /> By default, the reload() method simply reloads the specified page from the browser cache. This means that the page loads much faster; it also means that any changes to the page will not be reflected when it is reloaded. If you want to force the page to reload from the server, you need to specify a value of True for the Source attribute, as shown below: location.reload(true)<br /> <br /> By reloading the page from the server, you are ensured that the visitor will see the most current version of the page. The other method available with the Location object is the replace() method. This method allows you to change the page that displays when the user clicks the Back button. This may sound like an odd behavior, but have you ever visited a Web site where the page did not load properly when you clicked the Back button on the browser? When the Back button is clicked you can choose whether to display a customized page or the default page displays by using the following code: location.replace(www.microsoft.com/default.htm);<br /> <br /> When you use this method, the page you specify replaces the current page in the history list—that is, when the Back button is pressed, this is the page that will display. We are going to talk more about the window history in the next section.<br /> <br /> Working with Window History As you are probably aware, the browser window maintains a history of the pages that have been viewed during the current session. This information is stored in the History object. From the browser window you can scroll back and forth through the history list using the Back and Forward buttons. In the previous section, we looked at how you can modify the page that displays when the Back button is clicked. In addition, you can also use the methods associated with the History object to move through the history list. There are three different methods that you can use with the History object, as outlined in Table 12.5.<br /> <br /> 12<br /> <br /> 394<br /> <br /> Day 12<br /> <br /> TABLE 12.5<br /> <br /> History Object Methods<br /> <br /> Method<br /> <br /> Description<br /> <br /> back()<br /> <br /> Moves back through the history list from the current position. Simulates the Back button on the browser.<br /> <br /> forward()<br /> <br /> Moves forward through the history list from the current position. Simulates the Forward button on the browser.<br /> <br /> go()<br /> <br /> Moves forward or backward within the history list. A negative number moves backward through the list the specified number of pages and a positive number moves forward. If you specify a value of zero, the current page refreshes.<br /> <br /> Keep in mind that if the history does not exist in the direction you specify with these methods, the browser remains on the current page. As you can see from Table 12.5, the go() method provides the most flexibility by allowing you to jump to any page within the history list. For example, you can create your own back link to move back two pages in the history list, as follows: <a href=”javascript:void(0)” onclick=”history.go(-2)” rel="nofollow">Return to Main Page</a><br /> <br /> Again, this code causes the browser to display the page that displayed two pages prior to the current page. This allows you to customize how the user moves through the history. By the same token, you may want the movement within the list to display a specific page, which can be accomplished by specifying a URL. For example, the following code would jump to the default page for Microsoft: <a href=”javascript:void(0)” onclick=”history.go(http://www.microsoft.com) rel="nofollow"> Return to Home Page</a><br /> <br /> You need to keep in mind that if you specify a URL for the go() method, the URL must be within the history list or the browser will remain on the current page.<br /> <br /> Note<br /> <br /> Be aware that you cannot access URLs that are listed in the history list. You can simply scroll through the list by going back and forward.<br /> <br /> Working with the Status Line Earlier in this chapter, when the window chrome features were introduced that can be specified for the Window object, we looked at how to specify whether or not the status bar would display at the bottom of the window. The status bar normally contains messages<br /> <br /> Windows and Frames<br /> <br /> 395<br /> <br /> related to the page being viewed. For example, the status bar displays the URL when you drag your mouse cursor across a link. You also can place custom messages in the status bar by using the Status property of the Window object. To put a message in the status bar, you simply set the value of the status property equal to the message you want to display, as follows: status =”Welcome to our Web Site”;<br /> <br /> Notice that although the property is associated with the Window object, as we discussed at the beginning of the chapter, it is not necessary to use the Window object reference with the properties because JavaScript automatically assumes that the property belongs to the Window object. Keep in mind, the message will display on the status bar until a new message is assigned to the status bar, either by you or the browser. With that in mind, look at the code in Listing 12.3, which displays information about links in the status bar when the mouse cursor drags across the link.<br /> <br /> INPUT<br /> <br /> LISTING 12.3<br /> <br /> Changing the Status Bar Message (statusmsg.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function StatusMsg(LinkURL, LinkDesc) { if (LinkURL == “”) { status = “ “ } else { status = (“Visit “ + LinkDesc + “ at “ + LinkURL) } return true } //..> </script> </head> <body> <title>Change Status Message</title> </head> <body> <p><a href = “http://www.microsoft.com” onmouseover = “return StatusMsg(this.href, ‘Microsoft’)” onmouseout = “return StatusMsg(‘’,’’)” rel="nofollow">Microsoft</a></p><br /> <br /> 12<br /> <br /> 396<br /> <br /> Day 12<br /> <br /> LISTING 12.3<br /> <br /> continued<br /> <br /> <p><a href = “http://www.netscape.com” onmouseover = “return StatusMsg(this.href, ‘Netscape’)” onmouseout = “return StatusMsg(‘’,’’)” rel="nofollow">Netscape</a></p> <p><a href = “http://www.samspublishing.com” onmouseover = “return StatusMsg(this.href, ‘Sams Publishing’)” onmouseout = “return StatusMsg(‘’,’’)” rel="nofollow">Sams</a></p> </body> </html><br /> <br /> If you look at the code in Listing 12.3, you will notice that at the end of the StatusMsg function there is a return true statement. When using the status property with the onMouseOver event, you must use the return statement in order for your JavaScript to function properly. Although it is rare to use the return statement with your JavaScript code, in this instance it is necessary. When the user drags the cursor across a link the message in the status bar changes to describe where the link goes, as shown in Figure 12.4.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 12.4 You can change the status bar message using the status property.<br /> <br /> The code to change the status bar message is typically quite short and, therefore, it is typically included in only the <a href rel="nofollow"> tag. Doing so eliminates the need to create a separate JavaScript function. Although, if you do, you need to make sure you include both the code to set the status bar message and the return true statement discussed above. To do so, you need to separate each statement with a semicolon as illustrated in this code: <a href = “http://www.netscape.com” onmouseover = “window.status = ‘Visit Netscape at: ‘ + this.href ; return true” rel="nofollow"><br /> <br /> Windows and Frames<br /> <br /> 397<br /> <br /> When you open most pages the status bar is typically empty. This is because the default status bar message is blank. You can customize the default message that displays when other messages are not displayed by using the defaultstatus property. You set this property in the same fashion that the status property was set earlier: defaultstatus = “Welcome to this JavaScript site”;<br /> <br /> When you have the defaultStatus property specified in your code, the text specified for the property displays anytime another message is not displayed on the status bar. If another message displays, as soon as that message finishes, the default message displays again.<br /> <br /> Using the Screen Object In addition to the browser that the visitor to your site is using, the other issue to keep in mind is the screen resolution. Although most developers have learned to run high resolutions to provide more screen space, many visitors run much lower resolutions. So the main issue becomes determining at what screen size to code your page. If you go with the lowest common denominator, 640 x 480, everyone can view the site; however, you are greatly limited to the amount of stuff that you can place on the screen, and visitors with higher resolutions will see a lot of white space on the right side of the browser window. Luckily, there is a way to get the resolution information and adjust your code as desired.<br /> <br /> Note<br /> <br /> Keep in mind that creating a site that looks good at a higher resolution does not keep a lower resolution browser from visiting your site. It simply means that those visitors will not see as much on the page without scrolling.<br /> <br /> In order to get the resolution information for the visitor’s screen, you need to access the properties of the Screen object. All of these properties, listed in Table 12.6, are read-only and therefore you cannot change the properties; however, you can adjust your own HTML page code to fit within the parameters. TABLE 12.6<br /> <br /> Screen Object Properties<br /> <br /> Property<br /> <br /> Description<br /> <br /> availHeight<br /> <br /> Indicates the maximum height of the viewing monitor in pixels<br /> <br /> availWidth<br /> <br /> Indicates the maximum width of the viewing monitor in pixels<br /> <br /> colorDepth<br /> <br /> Indicates the bits per pixel<br /> <br /> 12<br /> <br /> 398<br /> <br /> Day 12<br /> <br /> TABLE 12.6<br /> <br /> continued<br /> <br /> Property<br /> <br /> Description<br /> <br /> height<br /> <br /> The actual height of the monitor in pixels<br /> <br /> width<br /> <br /> The actual width of the monitor in pixels<br /> <br /> The availHeight and availWidth measurements indicate the maximum size of the browser window. As you are aware, on most operating systems there are other elements that require part of the screen space, such as the toolbar on a Microsoft Windows operating system. For example, with the use of these properties you can create JavaScript code that automatically resizes the browser window to fill the screen, as follows: Window.moveTo(0,0); Window.resizeTo(screen.availWidth, screen.availWidth);<br /> <br /> Notice the resizeTo() method was used to change the size of the current browser window. We also used the moveTo() method because we wanted to move the browser window to the upper-lefthand corner of the screen before resizing it. That way, the window will appear to cover all available space. You can of course create code for different resolutions, and then the appropriate code based upon the screen settings. For example, you can use if statements to check screen resolution and then execute appropriate code, as shown in Listing 12.4.<br /> <br /> INPUT<br /> <br /> LISTING 12.4<br /> <br /> Checking Screen Resolution (checkresolution.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-if (screen.width == 640) { alert(“Resolution is 640 by 480”); } else if(screen.width == 800) { alert(“Resolution is 800 by 600”); } else if (screen.width == 1024) { alert(“Resolution is 1024 by 768”); } else if (screen.width > 1024) {<br /> <br /> Windows and Frames<br /> <br /> LISTING 12.4<br /> <br /> 399<br /> <br /> continued<br /> <br /> alert(“Resolution is greater than 1024 by 768”); } //..> </script> </head> <body> </body> </html><br /> <br /> Notice in Figure 12.5 that we display a different message depending upon the screen resolution. Remember, although 640 x 480 is considered to be the lowest resolution, there are probably going to be even higher resolution monitors developed which you may need to bear in mind as you write your code.<br /> <br /> OUTPUT FIGURE 12.5 You can use the Screen object’s properties to determine the resolution of the visitor’s monitor.<br /> <br /> 12<br /> <br /> As you can see, there are multiple methods you can use to set up your code to best utilize the visitor’s screen size. Again, this type of coding does not affect the ability of your JavaScript code to run, but it does affect how it appears on the screen.<br /> <br /> Working with Frames Since you are learning JavaScript, you probably have heard of frames—especially if you have done much work with HTML. Frames provide the ability to create the appearance of multiple windows in one browser window. In other words, by using frames you are able to display multiple pages in one browser window each within its own frame, and<br /> <br /> 400<br /> <br /> Day 12<br /> <br /> each frame is totally independent of the other frames in the browser window. The user of frames is illustrated in Figure 12.1, shown earlier in this chapter. Although frames were incredibly popular three or four years ago, they have lost their popularity among many Web developers and users. There are different reasons for this, in particular the difficulty of printing an entire Web page. Even so, they are still a great way to layout a Web page and maintain navigation buttons and other pertinent information in the same location on screen. For example, if you want a column of buttons displayed on the left side of the window that will remain there no matter what direction the user scrolls, frames are the only method to accomplish this. Before looking at how you can manipulate the contents of frames using JavaScript, let’s review some basic frame technology to clarify our discussion. In order to use frames you must first have an HTML page that defines the frame layout using the <frameset> tag. The <frameset> tag describes how the frames are laid out on the page, as shown in Listing 12.5.<br /> <br /> INPUT<br /> <br /> LISTING 12.5<br /> <br /> Defining the Frame Layout (framepage.htm)<br /> <br /> <html> <head> <title>Frame Page</title> </head> <frameset cols=”25%,75%”> <frame src=”frame1.htm” name=”frame1”> <frame src=”frame2.htm” name=”frame2”> </frameset> </html><br /> <br /> Each frameset defines the layout using multiple rows or columns. For example, in Listing 12.5 there are two columns and the first column takes up 25% of the browser window, while the second column takes the remaining 75%. You can also define the size of the row or column as an exact pixel size, or you can use * to let the browser set its size.<br /> <br /> ANALYSIS<br /> <br /> Note<br /> <br /> The layout of the frameset can become very complex in that you can create nested framesets. For example, you can create a frameset with two columns and then break one column into three rows.<br /> <br /> Windows and Frames<br /> <br /> 401<br /> <br /> Each frameset contains at least one <frame> tag that defines the contents of the frame. The frame always has a src attribute that specifies the URL of the page to display in the particular frame. There are also some other attributes that you can use when defining the frame, as outlined in Table 12.7. Later in this section we will discuss how you can also change the values of these attributes using JavaScript. TABLE 12.7<br /> <br /> Frame Attributes<br /> <br /> Property<br /> <br /> Description<br /> <br /> frameborder<br /> <br /> Specifies whether or not a border displays for the frame. A value of 1 displays the frame and 0 hides the frame.<br /> <br /> marginheight<br /> <br /> Indicates the pixel size of vertical margins within the frame.<br /> <br /> marginwidth<br /> <br /> Indicates the pixel size of the horizontal margins within the frame.<br /> <br /> name<br /> <br /> Indicates the name of the frame. This is needed so you can reference the frame within your JavaScript code.<br /> <br /> noresize<br /> <br /> A value of true allows the user to resize the frame; whereas, specify false if the frame should not be resized.<br /> <br /> scrolling<br /> <br /> Specifies whether scrolling is allowed in the frame. Specify a value of yes to allow scrolling; specify no, if scrolling should not be allowed, or auto to let the browser determine if scrolling is needed.<br /> <br /> src<br /> <br /> Indicates the URL of the file to display in the frame.<br /> <br /> Note<br /> <br /> If you look at the frameset and frames in a slightly different approach, you will see that the frameset is the parent window and each frame is a child window. Keep this in mind when we discuss referencing different frames.<br /> <br /> Unfortunately there are still instances where the user may have a browser that does not support frames. This is very possible with the many new mobile devices coming to market. If so, you need to have a way to catch and redirect those users to other code. This can be accomplished using the <noframes></noframes> tag pair. You add the <noframes> tag to your frameset definition so that the browser knows what command to execute if frames are not supported. <frameset cols=”25%,75%”> <frame src=”frame1.htm” name=”frame1”> <frame src=”frame2.htm” name=”frame2”> <noframes>Your browser does not support frames</noframes> </frameset><br /> <br /> 12<br /> <br /> 402<br /> <br /> Day 12<br /> <br /> Finally, let’s discuss one more frame element—the inline frame, which is indicated by the <iframe> tag.<br /> <br /> INPUT<br /> <br /> <iframe src = “http://www.microsoft.com”></iframe><br /> <br /> The inline frame is used to place a frame on an HTML page, as shown in Figure 12.6, which shows a single inline frame placed on an HTML page.<br /> <br /> OUTPUT FIGURE 12.6 You can use inline frames on HTML pages where you want to insert the contents of a single page.<br /> <br /> Keep in mind that you can use all of the attributes listed in Table 12.7 to customize the look of your inline frame. Now that we have covered the basics of working with frames, let’s look at the process of working with frames in JavaScript. As discussed at the beginning of this chapter, frames are essentially just windows and therefore you are going to see a lot of the same processes that were discussed early on in this chapter. As mentioned earlier, just like the other elements of the HTML page, frames have a treelike hierarchy. But, because frames are essentially just windows, when you use frames the frameset is the parent window and each frame it references is a child window. Actually, the parent window, or frameset, has a Frames property that contains an array of the child frames. By using the frames array, you can reference each frame in the order in which it is listed in the frameset with the first frame referenced as frame[0]. This being the case, you can reference a frame using either the frame name or the frames array. For example, look at the following frameset code:<br /> <br /> Windows and Frames<br /> <br /> 403<br /> <br /> <frameset cols=”25%,75%”> <frame src=”frame1.htm” name=”frame1”> <frame src=”frame2.htm” name=”frame2”> <noframes>Your browser does not support frames</noframes> </frameset><br /> <br /> From the frameset page, you would reference the first frame as either frame1 or frame[0]. From the child frame you can reference the parent window, or frameset, simply as parent. Probably the bigger issue becomes the ability to reference another child frame from the current child frame. In order to do this you will need to reference the parent window. So, with our sample code frame1, you can reference frame2 by one of the following two methods: parent.frame2 parent.frames[1]<br /> <br /> There are a couple of properties that you should keep in mind when dealing with frame references. The self property allows you to reference the current frame. For example, you can type self.parent to refer to the parent frame; or as was mentioned earlier, simply parent. In most cases, the self property is not needed, but using it helps clarify your point of reference. Another property that you need to keep in mind is the top property. This property refers to the top window object in the browser. This property comes in handy when you have nested loops because it allows you to back up to the top window and then specify your reference. Keep this hierarchical structure in mind as we look at changing properties of different frames in the rest of this section.<br /> <br /> Referencing Frame Locations As already discussed, you specify the location or URL of the page in order to display in a frame part of the frameset definition in HTML. <frame src=”frame1.htm” name=”frame1”><br /> <br /> The HTML definition of the frame contents works well for initially loading the window. Typically, however, you will want to update your Web pages dynamically by changing the content of one or more frames within your JavaScript code. As with all window objects, the URL information for each frame is stored in the Location object. Therefore, to change the contents of the current frame, you would specify the following: location = URL<br /> <br /> 12<br /> <br /> 404<br /> <br /> Day 12<br /> <br /> is the address of the page to display in the current frame. For example, to display another file on the same Web server, you would simply specify the filename, as follows:<br /> <br /> URL<br /> <br /> location = “menu2.htm”<br /> <br /> Although when dealing with frames, you typically will want to change the contents of another frame. For example, you may have a list of sites in one frame and when you click on a link the contents of the second frame change. If the frame containing the menu is the first frame, a change to the URL displayed in the second frame would be accomplished by typing the following: parent.frame[1].location = “http://www.netscape.com”<br /> <br /> Of course, if you know the name of the frame, you can also write this statement as follows, which uses frame2 as the second frame name: parent.frame2.location = “http://www.netscape.com”<br /> <br /> As seen earlier, when you only have one line of JavaScript code to execute, you can also add the code to the corresponding HTML tag. For example, you can specify the following code to change the contents of the second frame: <a href=”javascript:void(0)” onclick=”parent.frame[1].location = “http://www.netscape.com” rel="nofollow">Netscape</a><br /> <br /> As you can see, you can change the contents of a frame fairly simply, as long as you understand the tree-structure that JavaScript follows to locate each frame. When you use this structure, you can change one frame or multiple frames simultaneously, as shown in Listing 12.6.<br /> <br /> INPUT<br /> <br /> LISTING 12.6 Changing Multiple Frames (ChangeFrame1.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function changeLocation() { top.frame2.location.href = document.form1.NewURL1.value; top.frame3.location.href = document.form1.NewURL2.value; } //..> </script> </head> <body> <title>Specify URL</title> </head> <body><br /> <br /> Windows and Frames<br /> <br /> LISTING 12.6<br /> <br /> 405<br /> <br /> continued<br /> <br /> <form name=”form1”> Left Frame: <input type=”text” name=”NewURL1” size=”40”><br> Right Frame: <input type=”text” name=”NewURL2” size=”40”> <input type=”button” value=”Change URLs” name=”URLButton” onclick = “changeLocation()”></p> </form> </body> </html><br /> <br /> As with any frames, you must have a frameset page. For the code in Listing 12.6, we have a frameset page made up of two nested framesets, as shown in Listing 12.7.<br /> <br /> INPUT<br /> <br /> LISTING 12.7<br /> <br /> Nested Frames (Changeframesmain.htm)<br /> <br /> <html> <head> <title>Changing Frames</title> </head> <frameset rows=”20%,80%”> <frame src=”changeframe1.htm” name=”frame1”> <frameset cols = “50%,50%”> <frame src=”changeframe2.htm” name=”frame2”> <frame src=”changeframe3.htm” name=”frame3”> </frameset> </frameset> </html><br /> <br /> 12 Because the frames are nested, we refer to the frames with URL locations we want to change by starting with the top property to reference the very top window. The bottom two frames are updated to reflect the specified URLs, as shown in Figure 12.7. You can change the URL displayed in multiple frames simultaneously. As you can see, it is fairly easy to change the URL of the page displayed in each frame. You just need to be careful that you reference the frame correctly. One potential problem when referencing frames is the fact that another site can load your site within its own frames. To avoid any potential reference problems, you can create code that breaks your site out of another site’s frames, as described in the next section.<br /> <br /> 406<br /> <br /> Day 12<br /> <br /> OUTPUT FIGURE 12.7<br /> <br /> Breaking Out of Frames One common feature of many sites is to open another URL within existing frames. This process keeps the visitor on your site with a frame displaying the other URL. This is similar to what we did in Figure 12.7, where we displayed two different sites, Netscape and Microsoft, on the same page simultaneously, while the top frame displayed content from our own site. Although this is a cool feature, especially for the main site, it can become quite cumbersome, especially when you have more frames inside of those frames. Also, it can become a nuisance for your own JavaScript code to run within the frame of another site, because now when you reference the top frame, you are actually referencing a frame on another site. To eliminate any potential issues, you can add code to your site to break out of frames from the site that called your page. This code is fairly simple, and you can use it whether or not you are using frames on your site. In fact, by doing so, you can ensure that your site never appears as a frame in another site. To break out of frames from another site, you simply need to add the following code to the pages on your site. if (top != self) { top.location = self.location; }<br /> <br /> Windows and Frames<br /> <br /> 407<br /> <br /> Of course, this statement also will break up frames on your own site. Therefore, if you are using frames on your own site, you will need to modify the code slightly to ensure that your main frame page becomes the top page, as outlined in the next section.<br /> <br /> Forcing Frames Once you set up frames for your Web site, you will want to ensure that the frames always load when people visit your site. This typically isn’t an issue when they access your default page, but if they access your site using a link from a search page, it is possible that they will access a non-frameset page. To ensure that they see the frames, you can add code to the top of the page to ensure that the frames display. if (top == self) { top.location = “default.htm”; }<br /> <br /> The above code works great for ensuring that the visitor sees the main page of your Web site. More than likely though, they will lose the link to the page where they entered your site and now will be forced to relocate it. So, in an ideal world, you would design your code to open the frames and display the page that was specified when your site was accessed in the appropriate frame. Of course, this takes a little more complex coding, but once you place this code on each page your site maintains a consistent look and feel. For example, Listing 12.8 illustrates the type of code you would place in each frame page to call the main frameset page.<br /> <br /> INPUT<br /> <br /> LISTING 12.8<br /> <br /> Forcing Frames (loadframepage.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-if (top==self) { var main_frame = “mainframepage.htm”; var cur_url = self.location.href; var setframes = main_frame + “?” + cur_url; location.href = setframes; } //--> </script> </head> <body> <h1>Frame Page</h1> This page loads within Main Frame page. </body> </html><br /> <br /> 12<br /> <br /> 408<br /> <br /> Day 12<br /> <br /> Notice that we created a search string to pass the URL of the current page into the main frameset page. When the frameset page, illustrated in Listing 12.9, receives the search string it places the specified URL in the appropriate frame.<br /> <br /> INPUT<br /> <br /> LISTING 12.9<br /> <br /> Main Frame Page (mainframepage.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-var frame_URL; if (location.search) { frame_URL = location.search.substring(1); } else { frame_URL = “changeframe2.htm”; } { document.write(“<frameset rows = ‘20%, 80%’>”) document.write(“ <frame src=’frame1.htm’>”) document.write(“<frame src=’” + frame_URL + “‘>”) document.write(“ </frameset>”) } //..> </script> </head> <body> </body> </html><br /> <br /> The code on this page is designed so that the page can be accessed directly or from another page. In other words, if the page is not called from one of the pages on your site, a search string is not passed and the default page is opened in the second frame. Otherwise, the URL of the page that called the main frame page is opened in the second frame.<br /> <br /> ANALYSIS<br /> <br /> Now we should consider combining the code to break out of another site’s frames and to force the frames thereby ensuring that the visitor gets the ideal visit. You can do this by adding the code we looked at in the previous section to the top of your frameset page. This type of frame coding can get rather complex, especially if you start coding for nested frames. But once you master this type of coding you can ensure not only that frames display but also that you don’t get trapped in someone else’s frames.<br /> <br /> Windows and Frames<br /> <br /> 409<br /> <br /> Working with Popups Earlier in this chapter we looked at how you can create windows that are specific sizes and control which window chrome displays on the window. This works well for loading a new page of information and maintaining the current browser open window. In fact, popup windows are commonly used on Web sites for displaying advertisement-type messages. You can create these types of popup windows fairly easily. For example, the code in Listing 12.10 creates a popup window that displays the text “Good Morning!”<br /> <br /> INPUT<br /> <br /> LISTING 12.10<br /> <br /> Creating Popup Windows (popupwindow.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-function GreetingWin() { DisplayGreeting = window.open(“”, “_blank”, “toolbar=no, status=no, width=200,height=200”); greeting = “<b>Good Morning!</b>” DisplayGreeting.document.write(greeting); } //--> </script> </head> <body> <input type=”button” value=”Click Button” onclick=”GreetingWin()”> </body> </html><br /> <br /> As you can see in Figure 12.8, the code in Listing 12.10 creates a window that appears to popup as soon as the button is clicked on the page. You can also have windows that popup when a page loads, or when any other event occurs. Actually, the Window object provides three other built-in popup windows that you can use as needed for different situations. Each of these methods is easy to use, and also provides the ability to gather input from the user. In fact, we have already used the alert() method in several examples in the last few chapters to display popup messages. These three methods are outlined in Table 12.8.<br /> <br /> 12<br /> <br /> 410<br /> <br /> Day 12<br /> <br /> OUTPUT FIGURE 12.8 You can create windows that appear to popup when a particular event occurs.<br /> <br /> TABLE 12.8<br /> <br /> Window Methods for Creating Popups<br /> <br /> Method<br /> <br /> Description<br /> <br /> alert<br /> <br /> Displays a message to the user in a popup window. The window contains one OK button, which can be used to close the window.<br /> <br /> confirm<br /> <br /> Displays a message in a popup window with a Yes and a No button. You can get the user’s input and use it to determine which tasks to perform.<br /> <br /> prompt<br /> <br /> Displays a message in a popup window with a text field where the user can type a response. The popup window also includes an OK and a Cancel button.<br /> <br /> To illustrate the difference between these three different popup windows, you can view the code in Listing 12.11, which uses the prompt() method to request the user’s name, the confirm() method to verify, and then the alert()method to display a message.<br /> <br /> INPUT<br /> <br /> LISTING 12.11<br /> <br /> Using Window Object Popup Methods (popupmethods.htm)<br /> <br /> <html> <head> <script language=”JavaScript” type = “text/javascript”> <!-var username = prompt(“Type your name:”, “”); if (confirm(“Your name is: “ + username + “. Is that correct?”) == true ) { alert(“Hello “ + username); } else {<br /> <br /> Windows and Frames<br /> <br /> LISTING 12.11<br /> <br /> 411<br /> <br /> continued<br /> <br /> alert(“Hi”); } //--> </script> </head> </html><br /> <br /> As you can see, each of the three methods displays a different type of popup window. We used an if statement to determine how to respond based on the button selected in the confirm popup, shown in Figure 12.9. If the user selects the OK button, then the alert method displays “Hello” and his name; otherwise the alert method simply displays the message “Hi”.<br /> <br /> ANALYSIS<br /> <br /> OUTPUT FIGURE 12.9 You can use the alert(), confirm(), and prompt() methods as popups to display and gather values from the user.<br /> <br /> Summary We have looked at many different elements of the Window and Frame objects in this chapter. We found out that in reality a frame is just another window; unlike the original concept we discussed where the Window object is the main object in the browser, and all other objects are children. When you are using frames you have multiple Window objects. Your main frameset window is the parent window and all other frames are child windows. We discussed how properties and methods can be applied to the current window or frame by simply using the property or method without an object reference. We looked at objects associated with the Window object, such as the Location object that stores the URL information for the currently displayed page and the History object that maintains a list of the pages viewed during the current session.<br /> <br /> 12<br /> <br /> 412<br /> <br /> Day 12<br /> <br /> We looked at how the Screen object properties can be accessed to determine the resolution settings for the user’s browser. We also showed how you can customize the messages that display on the status bar for the window.<br /> <br /> Workshop In the workshop today, we will use the questions and exercises to review what you have learned in this chapter about working with windows and frames.<br /> <br /> Q&A Q. Why should I use the Location object? A. Every Window object has a corresponding Location object. This object maintains the URL or location information for the information displayed on the browser window. By changing the properties of the Location object, you can change the URL of the page that is being displayed. You can also use the methods associated with this object to reload the content displayed in the browser window. Q. Do I really need to know what the user’s resolution is? A. The Screen object provides information about the resolution of the user’s monitor. By determining the resolution used by the user you can execute resolution specific code so that your page displays optimized for that specific resolution. This means by knowing this information you can make sure you don’t try to display something larger than the user’s resolution settings. For example, if you have several frames on your page you need to make sure they can all display within the user’s resolution settings. Q. How do I access different frames on the page? A. The frames property creates a frames array that stores references to each frame. The frames references are added to the array in the order in which they are listed in the frameset. You can reference a particular frame using the corresponding reference in the array. For example, to reference the first frame on the page you would use frame[0].<br /> <br /> Quiz 1. How do you set the default status bar message that displays when no other messages are displayed on the status bar? 2. If you have the following frameset code, how would you reference and change the location for frame2 to display the page changeframe4.htm by making the call from the page displayed in frame1?<br /> <br /> Windows and Frames<br /> <br /> 413<br /> <br /> <html> <head> <title>Changing Frames</title> </head> <frameset rows=”20%,80%”> <frame src=”changeframe1.htm” name=”frame1”> <frame src=”changeframe2.htm” name=”frame2”> <frame src=”changeframe3.htm” name=”frame3”> </frameset> </html><br /> <br /> 3. What are the differences generated by the following popup methods: alert(), confirm(), and prompt()?<br /> <br /> Quiz Answers 1. You can customize the default status bar message using the defaultStatus property. By setting this property, your default message displays on the status bar whenever there is not another message that needs to display in that location. You set the default status message by simply assigning the text value to the property: defaultStatus = “text message”. 2. To change the URL displayed by frame2, you need to reference the parent or top frame and then call the frame. You can either reference the frame by its name or by using the Frames array. Either one of these answers is correct. parent.frame2.location = “changeframe4.htm” parent.frames[1].location = “changeframe4.htm”<br /> <br /> 3. The alert() method simply allows you to create a popup window that displays a message to the user. This method does not expect any response from the user. The confirm() method creates a popup window and expects the user to provide a confirmation by selecting the appropriate button. The prompt() method displays a popup window containing a textbox where the user can type a value to return to your code.<br /> <br /> Exercises 1. Create a Web page that requests the user’s name, and then displays a custom greeting on the status bar. 2. Create a Web page with at least two frames. One frame should be a navigation menu that lists available pages on your Web site. When the user clicks a page, the corresponding page displays in the second frame. Be sure to add code for browsers that don’t support frames.<br /> <br /> 12<br /> <br /> WEEK 2<br /> <br /> DAY<br /> <br /> 13<br /> <br /> Regular Expressions Make It Easier We looked at string manipulation and some simple validation of user input briefly in Chapter 6, “HTML Forms and the String Object,” using some of the String object’s methods. At that time we mentioned that the processing we could achieve could be very much enhanced by the use of regular expressions. This was not an overstatement. Regular expressions, properly used, are a very powerful tool for the manipulation of strings and for precise validation of form, and other, data. In this chapter, we will discover some of the important tasks regular expressions can perform and just how much work regular expressions can save you when programming in JavaScript. This chapter will teach you • Why regular expressions are useful • What regular expressions are • How regular expressions are created • Several ways of using regular expressions<br /> <br /> 416<br /> <br /> Day 13<br /> <br /> Why Regular Expressions Are Useful There are many times when you might benefit from using regular expressions. However, if you were unaware of their existence, or unaware of how to use them, you could spend a lot of time creating a custom piece of programming to achieve an end that an appropriate regular expression could achieve much more quickly and much more easily. Let’s think about some real-world Web programming situations where you might find a regular expression useful. Suppose you have an e-commerce site where you want a customer to enter a credit card number, and you want to check if the credit card number is valid before you send it and the other form data to the server. One common structure for a credit card number is four groups of numerical digits, each of the four-digit groups with a space between each group. So, if you chose to have the user input in that format, the credit card number might look like the following: 1234 5678 9012 3456<br /> <br /> If you wanted to conduct client-side validation of a credit card number how could you go about it? Your first thought might be to treat the credit card number as a large integer, but integers don’t allow spaces between the numerical digits. Perhaps you might think of treating the credit card number as a string. If you treated it as a string how, with the JavaScript you have seen so far, would you be able to check the structure of the string? In theory you might be able to create a new function that checks each group of digits and whether or not they are in the range 0 to 9. To achieve that you would need many nested if and else statements. The logic would be fairly complex, and you would need to conduct extensive testing on your new code to make sure that you had allowed for all possible permutations of how a customer might enter a card number incorrectly. Perhaps, after a lot of effort, you may have created a script which will correctly process credit card numbers that consist of four groups of four digits separated by space characters. Quite possibly soon after you think you have finished that task and the site goes online customer complaints flood in because some customers want to enter their credit card number as 16 consecutive digits without any spaces. And, perhaps you succeed in writing yet more if and else statements to enable the script to handle both possibilities. However, just when you are feeling proud of your achievements a manager comes along and tells you that not all credit cards are 16 digits and that you also need to allow for credit card numbers of different lengths. What do you do? Your code is getting pretty complex already. Oh, and remember to add in debit cards, which have yet another set of number constraints. At that stage, you could be forgiven for regretting the day you got involved with JavaScript Web programming at all.<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 417<br /> <br /> Or suppose, for some confidential employment site, you need to collect personal identifiers such as a United States social security number (SSN) or a United Kingdom SSN, since your company has offices in both countries. How do you handle that? An American SSN is wholly numerical, while a UK SSN starts with two letters, has six numeric digits (usually in three groups of two), and a final letter. Adding code to check for one or both of those to the code you have created to accept credit card details is an intimidating task, particularly if you are not yet fully up to speed with JavaScript. Regular expressions are designed to solve programming problems exactly like this. What regular expressions do is transfer part of the burden of processing text structures (similar to the ones we have just considered) from you (the JavaScript programmer) to a regular expressions processor within the JavaScript interpreter. When you use regular expressions the burden of writing the JavaScript code, and working out the logic it represents, is lifted from you. All you need to do is be capable of expressing exactly what patterns of string you designate as allowable. The JavaScript interpreter takes care of the how of making sure that the designated pattern is allowed and all undesirable patterns are disallowed. Not surprisingly, a JavaScript interpreter must be given a very precise description of which patterns, or regular expressions, you want it to accept. So, let’s move on and consider what a regular expression is and how you can express the idea of a string pattern using JavaScript.<br /> <br /> What Is a Regular Expression? A regular expression, if it were expressed in plain English, is a description of a pattern of characters in a string. If we tried to describe what we wanted for the credit card example, we might say: “We want a pattern of four numeric digits, followed by an optional space character, followed by four numeric digits, followed by an optional space character, followed by four numeric digits, followed by an optional space character, which is then followed by four numeric digits.” As you can see it takes many words and quite a bit of space to express what we want. A regular expression does the same thing. It is simply a collection of special characters which represent a pattern. A regular expression is much more compact than saying the same thing in English. Saving space is, in one sense, a good thing; but saving space involves using symbols, which can get in the way of understanding until you become thoroughly familiar with them. So we will look first at very simple regular expressions, and then build up to more complex (and more useful) regular expressions.<br /> <br /> 13<br /> <br /> 418<br /> <br /> Day 13<br /> <br /> The pattern in a regular expression can be used to search a string for anything that matches the regular expression. The search could be for something very simple such as a certain name or number but simple tasks like that also could be done using the indexOf() method of the String object. The real power of regular expressions is the way they allow you to match something that is a pattern of characters rather than a literal sequence of characters. For example, we can use methods of the String object to look for literal strings such as “ABC”. But the String object’s methods don’t allow us to carry out tasks such as “Search for three uppercase characters among A, B and C” which is a pattern that a regular expression could deal with easily. Many scripters when they look at regular expressions for the first time find them very intimidating. This is probably because regular expressions look for a pattern, which is something that many scripters haven’t had to program for in their previous experience. So the syntax is very unfamiliar. Regular expressions can, at first glance, appear to be an impenetrable mish-mash of seemingly random characters. Although we can’t deny that the more advanced uses of regular expressions can be pretty complex, simpler regular expressions are fairly easy to understand and write once you grasp the basics of the syntax. We are going to take them one step at a time. Hopefully, you will gradually pick up exactly how they work. Let’s look at what is arguably the simplest type of regular expressions—matching a sequence of literal characters.<br /> <br /> Matching a Literal Character Sequence As well as matching more abstract or complex patterns, JavaScript regular expressions also allow us to match single literal characters or sequences of literal characters. For example, if you wanted to assign a JavaScript regular expression that matches the single literal alphabetic character “a” to the variable myRegExp, you could write this: var myRegExp = /a/ ;<br /> <br /> This syntax is similar to the assignment of a string to a variable: var myString = “a” ;<br /> <br /> When assigning a string, you use an opening quote mark, then the string, and then a closing quote mark. When you use a regular expression, you use an opening forward slash, /, followed by the pattern (in the example just shown, a single literal character), followed by a closing forward slash. All the text, or regular expression syntax (which we will look<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 419<br /> <br /> at soon), between the opening / and the closing / defines the pattern that the regular expression matches. Listing 13.1 shows a simple use of a simple regular expression in a character-guessing task.<br /> <br /> INPUT<br /> <br /> LISTING 13.1 The Regular Expression Matches a Single Lowercase “a” Only (SingleCharGuess.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Matching a single literal character</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsMatchingChar(str){ var myRegExp = /a/ ; return myRegExp.test(str) } // end function IsMatchingChar() function TestGuess(){ var guess = prompt(“Enter a lower case character here and see if it matches”, “It must be a lower case character”); if (IsMatchingChar(guess)){ alert(“You guessed correctly. The correct answer was “ + guess); } // end if else{ alert(“Sorry “ + guess + “ doesn’t match”); } // end else } // --> </script> </head> <body> <h3>This page allows you to guess a single literal lower case character</h3> <form> <button type=”Button” onclick=”TestGuess()”>Click here to guess</button> </form> </body> </html><br /> <br /> In Listing 13.1 the user is invited, via a prompt box, to enter a single character to see whether it matches the character chosen by the program, in this example the single lowercase character “a”.<br /> <br /> ANALYSIS<br /> <br /> 13<br /> <br /> 420<br /> <br /> Day 13<br /> <br /> The default behavior of a regular expression is that the pattern is case sensitive. Thus if you run Listing 13.1 you will find that you can enter the single lowercase character “a” and receive the message that you guessed the correct character. However, if you enter the uppercase character “A”, you will receive a message that you guessed incorrectly, just as you would if you had entered a “b”, a “c”, or some other character that does not match the pattern /a/. We can use longer literal strings using the same syntax. Thus, we could use the following in a Web page that asked the user to guess the FBI’s most wanted man: var mostWanted = /Osama Bin Laden/ ;<br /> <br /> However, we also might want to allow for people to make a guess using the string, “Osama bin Laden” (lowercase for the initial letter of “bin”, since the default behavior is that the pattern is case sensitive. We specify alternate literal strings of characters like this: var mostWanted = /Osama Bin Laden|Osama bin Laden/ ;<br /> <br /> The pipe character, “|”, allows you to specify two or more options for a literal string that matches the regular expression. Listing 13.2 shows how you can test for a long string of the type just shown and accept two possible literal strings as correct matches. LISTING 13.2<br /> <br /> INPUT<br /> <br /> Matching on More Than One Allowed Literal String (MostWanted.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>FBI Most Wanted Person</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsMatchingPerson(str){ var myRegExp = /Osama Bin Laden|Osama bin Laden/ ; return myRegExp.test(str) } // end function IsMatchingPerson() function TestGuess(){ var guess = prompt(“Enter a name here and see if it matches”, “It must be a full name”); if (IsMatchingPerson(guess)){ alert(“You guessed correctly. “ + guess + “ was a correct answer”);<br /> <br /> Regular Expressions Make It Easier<br /> <br /> LISTING 13.2<br /> <br /> 421<br /> <br /> continued<br /> <br /> } // end if else{ alert(“Sorry “ + guess + “ doesn’t match”); } // end else } // end function IsMatchingPerson() // --> </script> </head> <body> <h3>This page allows you to guess the FBI’s most wanted person in Quarter 4 2001</h3> <form> <button type=”Button” onclick=”TestGuess()”>Click here to guess</button> </form> </body> </html><br /> <br /> If you run Listing 13.2 you will find that there are two, and only two, ways you can guess correctly. One correct string is “Osama Bin Laden” and the other is “Osama bin Laden”. If you try to enter a first name only, you will be told that there is no match, or if you try the alternate spelling “Usama Bin Laden” that won’t match either.<br /> <br /> ANALYSIS<br /> <br /> Be careful that you don’t leave a space character after the pipe character within the variable assignment: var myRegExp = /Osama Bin Laden| Osama bin Laden/ ;<br /> <br /> Adding an extraneous space character means that the JavaScript interpreter will only match the second string if it begins with a literal space character. Thus, when you provide multiple options using the literal syntax you must avoid space characters around the pipe character (unless, of course, you really want space characters to be there). This means that a list of many options can be quite difficult to read, for example: var myRegExp = /Option1|Option2|Option3|Option4|Option5/ ;<br /> <br /> Caution<br /> <br /> JavaScript regular expressions behave a little differently from patterns in W3C XML Schema, for example. If you enter a space character at the beginning of the text in the prompt box, then a match will be declared. In a W3C XML Schema pattern that would not be a match because a space character does not match an initial uppercase “O”. JavaScript appears to strip whitespace at the beginning of a literal string.<br /> <br /> 13<br /> <br /> 422<br /> <br /> Day 13<br /> <br /> Some Simple Patterns Let’s move on to look at some simple patterns using regular expressions. First let’s look at the simple case where we want to offer a choice of an uppercase or lowercase character.<br /> <br /> Single Literal Character Choices As you saw a moment ago, you could do this using the pipe character. So, to define a pattern for uppercase or lowercase “A” or “a” you could use the following code: var myRegExp = /a|A/ ;<br /> <br /> For the following example, let’s use an alternate syntax to express the same choice: var myRegExp = /[aA]/;<br /> <br /> The square brackets express a choice of characters. In Listing 13.3, we use that syntax to allow the choice between an uppercase “A” and a lowercase “a”. In a moment we will come to the advantages of the square bracket syntax.<br /> <br /> INPUT<br /> <br /> LISTING 13.3 Allowing the Choice of Case in the Guess of a Single Character (SingleCharGuess2.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Matching a single literal character</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsMatchingChar(str){ var myRegExp = /[aA]/ ; return myRegExp.test(str) } // end function IsMatchingChar() function TestGuess(){ var guess = prompt(“Enter a character here and see if it matches”, “It can be either a lower or upper case character”); if (IsMatchingChar(guess)){ alert(“You guessed correctly. The correct answer was “ + guess); } // end if else{ alert(“Sorry “ + guess + “ doesn’t match”); } // end else }<br /> <br /> Regular Expressions Make It Easier<br /> <br /> LISTING 13.3<br /> <br /> 423<br /> <br /> continued<br /> <br /> // --> </script> </head> <body> <h3>This page allows you to guess a single literal character of either case</h3> <form> <button type=”Button” onclick=”TestGuess()”>Click here to guess</button> </form> </body> </html><br /> <br /> If you enter an “a” or “A” you will get a message indicating you successfully chose the correct matching character. If you choose other single characters, you will receive a message indicating that you made an incorrect guess.<br /> <br /> ANALYSIS<br /> <br /> If you tried to enter a multi-character string beginning with the correct character, then you will get a message (erroneously) indicating that you made a correct choice. Why is that? The reason behind the apparently incorrect behavior is that length of a string is an independent characteristic from the pattern. In other words, a pattern seeks a match for itself but also accepts longer matches which begin with the stated pattern. So, if you want to match a pattern and a length at the same time, you would need to test for the string length as well as whether it matches the regular expression. In the next example, we will add that length “error” trapping routine. First let’s look at how we can use the square brackets syntax in more complex examples. Suppose you wanted to ensure that the identifier for a machinery part started with an alphabetic character. How could you do that? You could use the pipe character and add a total of 52 choices (assuming that all alphabetic characters were valid). The square bracket syntax allows a much more compact way of expressing those multiple choices. For example, you could write the following: var myRegExp = /[a-zA-Z]/ ;<br /> <br /> What that means is that any character starting with lowercase “a” through lowercase “z” (expressed by the characters a-z in the pattern) or starting at uppercase “A” through uppercase “Z” (expressed by the characters A-Z in the pattern) would be acceptable. In Listing 13.4, we will allow this to be tested, as well as testing for the desired length (in this case one character).<br /> <br /> 13<br /> <br /> 424<br /> <br /> Day 13<br /> <br /> INPUT<br /> <br /> LISTING 13.4<br /> <br /> Allowing Any Alphabetic Character in a String of OneCharacter Length (AlphabeticOne.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Matching a single literal character</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsMatchingChar(str){ var myRegExp = /[aA]/ ; return myRegExp.test(str) } // end function IsMatchingChar() function TestGuess(){ var guess = prompt(“Enter a character here and see if it matches”, “It can be either a lower or upper case character”); if (IsMatchingChar(guess) && (guess.length==1)){ alert(“You guessed correctly. The correct answer was “ + guess); } // end if else if (guess.length!==1){ alert(“Sorry you entered a character string of the wrong length”); } else { alert(“Sorry “ + guess + “ doesn’t match”); } // end else } // --> </script> </head> <body> <h3>This page allows you to guess a single literal character of either case</h3> <form> <button type=”Button” onclick=”TestGuess()”>Click here to guess</button> </form> </body> </html><br /> <br /> Notice the if, else if and else statements in the TestGuess() function. If the text entered via the prompt box matches the pattern in the myRegExp variable and is the correct length (one character), then a message is displayed indicating success. If the length is incorrect, then that is indicated, no matter what the first character may be. If you were using the pattern to test whether a string contained rather than consisted of the pattern, then you would omit that part of the code. If the length is correct and the chosen character is incorrect, then that is indicated.<br /> <br /> ANALYSIS<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 425<br /> <br /> We can use the same technique to allow the two options for the spelling of the name “Osama Bin Laden”. A suitable regular expression is /Osama [Bb]in Laden/.<br /> <br /> Choices Using Sequences of Literal Characters Now let’s take this one step further. Suppose we had a catalog of parts that used an uppercase alphabetic character as the first character and a single numeric character as the second character. If we used the pipe syntax, we would need 260 choices explicitly expressed. However, by using the square bracket syntax we need only write the following: var myRegExp = /[A-Z][0-9]/<br /> <br /> The syntax [A-Z] indicates that the first character is an uppercase character between A and Z. The syntax [0-9] indicates that any numeric character from 0 to 9 inclusive is acceptable as the second character. You can try this out in Listing 13.5. Remember that we now are accepting a two-character parts code such as A4, C9, or W3.<br /> <br /> Note<br /> <br /> INPUT<br /> <br /> When we group possible characters using syntax such as [A-Z] we are creating a new character class. In JavaScript character classes are not named. However, it is reasonable, for example, to refer to [A-Z] as the uppercase alphabetic character class or to refer to [0-9] as the numeric digits character class.<br /> <br /> LISTING 13.5<br /> <br /> Combining Simple Character Classes in a Regular Expression Pattern (PartsCode.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Matching a sequence of literal characters</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsMatchingCode(str){ var myRegExp = /[A-Z][0-9]/ ; return myRegExp.test(str) } // end function IsMatchingCode() function TestGuess(){ var guess = prompt(“Enter a two character parts code here”, “Enter an upper case character followed by a number”); if (IsMatchingCode(guess) && (guess.length==2)){<br /> <br /> 13<br /> <br /> 426<br /> <br /> Day 13<br /> <br /> LISTING 13.5<br /> <br /> continued<br /> <br /> alert(“You chose a valid parts code: “ + guess); } // end if else if (guess.length!==2){ alert(“Sorry parts codes must be two characters long. Try again.”); } else { alert(“Sorry “ + guess + “ doesn’t match a valid parts code.”); } // end else } // --> </script> </head> <body> <h3>This page allows you to choose a two character parts code</h3> <form> <button type=”Button” onclick=”TestGuess()”>Click here to guess</button> </form> </body> </html><br /> <br /> In order to elicit the message that a string entered was correct the string must be exactly two characters in length, and it must have an uppercase alphabetic character as its first character and a numeric character as its second character.<br /> <br /> ANALYSIS<br /> <br /> We could use a similar approach if we wanted only characters A to M inclusive as the first character. In which case, we would define a new character class that contains only the uppercase alphabetic characters from A to M inclusive. The following code would express that: var myRegExp = /[A-M][0-9]/ ;<br /> <br /> Patterns That Use Numbered Occurrences of Classes Let’s move on and make it just a little more complex. Suppose we have a parts catalog that has a structure of two uppercase alphabetic characters, a dash, and three numeric characters. We can express that using the following syntax: var myRegExp = /[A-Z]{2}-[0-9]{3}/ ;<br /> <br /> The syntax [A-Z] indicates, as before, a choice from a character class of uppercase alphabetic characters. The syntax {2} indicates that we have a sequence of exactly two such uppercase alphabetic characters at the beginning of the pattern. The single “-” character indicates a literal dash. The syntax [0-9] indicates a choice from the numeric characters 0 to 9 inclusive, and the {3} indicates that we have exactly three such numeric characters.<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 427<br /> <br /> In Listing 13.6 you can test this six-character code.<br /> <br /> INPUT<br /> <br /> LISTING 13.6 A Pattern Combining Numbers of Characters from Defined Character Classes (SixCharCode.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Matching sequences of more than one literal character</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsMatchingCode(str){ var myRegExp = /[A-Z]{2}-[0-9]{3}/ ; return myRegExp.test(str) } // end function IsMatchingCode() function TestGuess(){ var guess = prompt(“Enter a six character parts code here”, “The format is two upper case letters, a dash and three numbers.”); if (IsMatchingCode(guess) && (guess.length==6)){ alert(“You chose a valid parts code: “ + guess); } // end if else if (guess.length!==6){ alert(“Sorry parts codes must be six characters long. Try again.”); } else { alert(“Sorry “ + guess + “ doesn’t match a valid parts code.”); } // end else } // --> </script> </head> <body> <h3>This page allows you to choose a six character parts code</h3> <form> <button type=”Button” onclick=”TestGuess()”>Click here to guess</button> </form> </body> </html><br /> <br /> This technique of using numbers of characters from defined character classes begins to open up solutions to some of the problems that were discussed earlier in this chapter.<br /> <br /> ANALYSIS<br /> <br /> Let’s look again at the 16-digit credit card number problem. The digits we use are all in the character class represented by [0-9]. We saw from Listing 13.6 that we can mix<br /> <br /> 13<br /> <br /> 428<br /> <br /> Day 13<br /> <br /> sequences of characters with literal characters. So we want to express a character class of numeric characters (of which there should be four) followed by a single space character, followed by four numeric digits, followed by a single space, followed by four numeric digits, followed by a single space, which is then followed by four numeric digits. Using a regular expression we can express that as follows: var myRegExp = /[0-9]{4} [0-9]{4} [0-9]{4} [0-9]{4}/ ;<br /> <br /> The syntax [0-9]{4} expresses the character class of numeric digits. The second part of the sub-expression, {4}, indicates that we use four successive characters from the numeric digits character class. If we want to use that pattern to test a credit card number, then we must remember to check that the length is 19 characters long exactly. So Listing 13.7 will allow us to get part way to solving the credit card validation problem that we discussed earlier.<br /> <br /> INPUT<br /> <br /> LISTING 13.7 Validating a 16-Digit Credit Card Number Separated by Single Space Characters (CreditCard01.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Matching a 16 digit credit card number separated by a single space</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsMatchingCard(str){ var myRegExp = /[0-9]{4} [0-9]{4} [0-9]{4} [0-9]{4}/; return myRegExp.test(str) } // end function IsMatchingCard() function TestGuess(){ var cardNo = prompt(“Enter a credit card number here”, “The format is 4 numbers, a space, 4 numbers, a space, 4 numbers, a space, 4 numbers”); if (IsMatchingCard(cardNo) && (cardNo.length==19)){ alert(“You entered a valid credit card number: “ + cardNo); } // end if else if (cardNo.length!==19){ alert(“Sorry you entered a credit card number of the wrong length. Try again.”); } else { alert(“Sorry “ + cardNo + “ isn’t a valid credit card number.”); } // end else }<br /> <br /> Regular Expressions Make It Easier<br /> <br /> LISTING 13.7<br /> <br /> 429<br /> <br /> continued<br /> <br /> // --> </script> </head> <body> <h3>This page allows you to enter and check a 16 digit credit card number</h3> <form> <button type=”Button” onclick=”TestGuess()”> Click here to enter card number</button> </form> </body> </html><br /> <br /> You also can use a similar technique to handle zip codes. To test for a standard zip code you could use the following syntax: var myRegExp = /[0-9]{5}/ ;<br /> <br /> The pattern indicates that five numeric digits make up the pattern. If we want to accept zip codes only in the extended zip code format, we can use the following pattern: var myRegExp = /[0-9]{5}-[0-9]{4}/ ;<br /> <br /> The pattern indicates that there are five numeric digits, followed by a single literal dash followed by four numeric digits. To accept both standard and extended zip codes, we would use the pipe character we used earlier, as shown here: var myRegExp = /[0-9]{5}-[0-9]{4}|[0-9]{5}/ ;<br /> <br /> Similarly, we can also now solve the United States SSN problem. In that case, we have three digits followed by a dash followed by two digits followed by a dash followed by four digits. So the following code would meet our need: var myRegExp = /[0-9]{3}-[0-9]{2}-[0-9]{4}/ ;<br /> <br /> If we want to be able to accept the format using dashes as well as accept the format simply using a sequence of digits we could use the following syntax, with the pipe character separating the two acceptable formats: var myRegExp = /[0-9]{3}-[0-9]{2}-[0-9]{4}|[0-9]{9}/ ;<br /> <br /> Not all the problems we might want to solve use a fixed numbers of characters chosen from particular sequences of characters. So let’s move on to examine how we can use<br /> <br /> 13<br /> <br /> 430<br /> <br /> Day 13<br /> <br /> (within a pattern) variable numbers of characters which are chosen from defined character classes that don’t necessarily use sequences of characters.<br /> <br /> Patterns Using Variable Numbers of Occurrences We have made much progress in solving the problems discussed earlier. By adding syntax to allow for variable numbers of characters, we can add yet more power and flexibility to our growing repertoire of regular expressions. As a next step, let’s look at how we can allow for a user entering a credit card number to accidentally or deliberately omit one or more spaces. In Listing 13.7, omitting any spaces would result in a message that the credit card number entered was not valid. We need a syntax that allows for zero or one space character between the groups of four numeric digits. The following syntax allows the user to omit one or more of the space characters: var myRegExp =<br /> <br /> /[0-9]{4} {0,1}[0-9]{4} {0,1}[0-9]{4} {0,1}[0-9]{4}/;<br /> <br /> The only difference in this code from that used in Listing 13.7 is that rather than having a single space character with the unexpressed default count of 1, we use the curly brackets syntax in a new way, {0,1}, to indicate that the literal space character may occur a minimum of zero times and a maximum of one time. Since we are now allowing the user to possibly omit spaces, we cannot continue to use a simple fixed length for the credit card number string. The shortest possible length is 16 characters (when all spaces are omitted), and the maximum length is 19 characters (when all single space characters are present). Listing 13.8 will show you how to implement this new flexibility on the regular expression pattern and length. It will still report an error when a user enters an alphabetic character, or if a user inserts a space anywhere but between the allowed four numeric characters from the defined character class.<br /> <br /> INPUT<br /> <br /> LISTING 13.8<br /> <br /> Allowing Variable Whitespace in a Credit Card Number (CreditCard02.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Matching a 16 digit credit card number with variable number of space characters</title> <script type=”text/javascript” language=”javascript”><br /> <br /> Regular Expressions Make It Easier<br /> <br /> LISTING 13.8<br /> <br /> 431<br /> <br /> continued<br /> <br /> <!-- // function IsMatchingCard(str){ var myRegExp = /[0-9]{4} {0,1}[0-9]{4} {0,1}[0-9]{4} {0,1}[0-9]{4}/; return myRegExp.test(str) } // end function IsMatchingCard() function TestGuess(){ var cardNo = prompt(“Enter a credit card number here”, “The format is 4 numbers, a space, 4 numbers, a space, 4 numbers, a space, 4 numbers”); var validLength = ((cardNo.length>=16) && (cardNo.length<=19)); if (IsMatchingCard(cardNo) && (validLength)){ alert(“You entered a valid credit card number: “ + cardNo); } // end if else if (!validLength){ alert(“Sorry you entered a credit card number of the wrong length. Try again.”); } else { alert(“Sorry “ + cardNo + “ isn’t a valid credit card number.”); } // end else } // --> </script> </head> <body> <h3>This page allows you to enter and check a 16 digit credit card number</h3> <form> <button type=”Button” onclick=”TestGuess()”>Click here to enter card number</button> </form> </body> </html><br /> <br /> If you run Listing 13.8 and check the various possibilities that we have allowed the user, you will find that we have permitted any combination of zero or one space characters at the appropriate places. In addition, perhaps surprisingly, you will find that the user can now enter a leading space character and be told that the number is valid if one or more of the later space characters are omitted.<br /> <br /> ANALYSIS<br /> <br /> We can also apply the variable frequency technique to situations where we may need to allow variable occurrences of a character class. Suppose we have a parts catalog where the parts numbers can be of the forms A12 or AB12. We can express that using the following code: var myRegExp = /[A-M]{1,2}[0-9]{2}/ ;<br /> <br /> 13<br /> <br /> 432<br /> <br /> Day 13<br /> <br /> The sub-expression [A-M] defines a character class and its permitted occurrences as {1,2}. Expressed in plain English, uppercase alphabetic characters from “A” to “M” inclusive, which may occur once or twice, are allowed. Another time when you might want to use a pattern is where you want to allow variable length occurrences of a character class to check whether an e-mail address entered by a user is or is not valid. Let’s pause and think how an e-mail address is made up. First let’s look at the simple form structure of an e-mail address: consultancy@xmml.com<br /> <br /> There is a sequence of alphabetic characters followed by a literal “@” character followed by alphabetic characters representing the domain name, followed by a period, which is then followed by a sequence of characters representing such entities as “com”, “net,” and so on. We need to remember that numeric characters are also allowed in the part that is to the left of the @ sign and in the domain name. Additionally, in the domain name, a hyphen character is allowed. However, remember that the underscore character is not allowed in a domain name. There is an additional issue about whether we allow and test the pattern for upper- and lowercase characters or only lowercase characters. The solution shown here uses only lowercase alphabetic characters. In parallel with that in the listing that follows, we will use a method of the String object to convert the e-mail address to lowercase characters before testing the pattern. Finally, how many characters should to be allowed? The maximum number of characters permitted for a domain name is 65, as shown here: var myRegExp = /[a-z0-9-]{1,30}@[a-z0-9-]{1,65}.[a-z]{3}/ ;<br /> <br /> Listing 13.9 is an example of the e-mail checking routine in place. We need to check for a minimum length since an e-mail address such as j@j.net is the shortest allowed by the regular expression pattern. Thus, we define the validLength variable as being greater than or equal to 6 characters in length.<br /> <br /> INPUT<br /> <br /> LISTING 13.9<br /> <br /> Checking an E-Mail Address, Version 1 (CheckEmail01.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html><br /> <br /> Regular Expressions Make It Easier<br /> <br /> LISTING 13.9<br /> <br /> 433<br /> <br /> continued<br /> <br /> <head> <title>Checking an email address - Version 01</title> <script type=”text/javascript” language=”javascript”> <!-- // function IsMatchingAddress(str){ var myRegExp = /[a-z0-9-]{1,30}@[a-z0-9-]{1,65}.[a-z]{3}/ ; return myRegExp.test(str) } // end function IsMatchingAddress() function TestGuess(){ var EmailAddr = new String(); entry = prompt(“Enter an email address here”, “Please type it carefully”); EmailAddr = entry; EmailAddr = EmailAddr.toLowerCase(); var validLength = (EmailAddr.length>=6); if (IsMatchingAddress(EmailAddr) && (validLength)){ alert(“You entered a valid email address: “ + EmailAddr); } // end if else if (!validLength){ alert(EmailAddr.length); alert(“Sorry you entered an email address of the wrong length. Try again.”); } else { alert(“Sorry “ + EmailAddr + “ isn’t a valid email address.”); } // end else } // --> </script> </head> <body> <h3>This page allows you to enter and check an email address such as consultancy@xmml.com</h3> <form> <button type=”Button” onclick=”TestGuess()”> Click here to enter email address</button> </form> </body> </html><br /> <br /> The code in Listing 13.9 will pick up many possible errors in an e-mail address.<br /> <br /> ANALYSIS It will, for example, limit domain names to alphabetic characters, numeric characters, and a dash (hyphen) in accordance with the domain-naming conventions. However, it has limitations in a number of respects. As currently written, it will incorrecetly indicate an error for the valid (but fictional) e-mail address of john.doe@samspublishing.com.<br /> <br /> 13<br /> <br /> 434<br /> <br /> Day 13<br /> <br /> It will also accept as its final three characters, any three alphabetic characters. So sleazy@dubious.xxx would wrongly be accepted as a valid e-mail address. With the advent of domains such as .info a pattern of three alphabetic characters is no longer comprehensive, even for top-level domains. In addition, country-level domains that may be used in an e-mail address would wrongly be rejected as invalid. If you want to be sure that no valid e-mail addresses are incorrectly rejected, then you could opt for the simple but safe solution, which basically checks for an “@” character and allows almost anything else, like so: var myRegExp = /[a-z0-9-.]{1,40}@[a-z0-9-.]{4,70}/<br /> <br /> This regular expression allows alphanumeric characters plus a dash and a period in any sequence from one to 40 characters in length. This is followed by an “@” character, which in turn is followed by a minimum of 4 characters (for possible country-level domains such as a .tv) up to 70 characters in length to allow for 65-character domain names followed by a period, followed by four character domains such as “.info”. Alternatively, we can attempt to list the current (for the moment) global top-level domains, com, net, org, info, and biz, and add further options to cover country-level domains too. The following regular expression lists, within parentheses, the global toplevel domains just mentioned, separated by the pipe character, offers additional options: var myRegExp = /[a-z0-9-.]{1,30}@[a-z0-9-]{1,65} .(com|net|org|info|biz|([a-z]{2,3}.[a-z]{2}))/ ;<br /> <br /> The following would match the regular expression: Andrew.Watt@XMML.com // matches literal “com” Andrew.Watt@XMML.co.uk // matches [a-z]{2,3}.[a-z]{2} Andrew.Watt@XMML.org.uk // matches [a-z]{2,3}.[a-z]{2} Andrew.Watt@XMML.net // matches literal “net”<br /> <br /> Listing 13.10 uses the more precise test for an e-mail address to allow you to explore this issue further.<br /> <br /> INPUT<br /> <br /> LISTING 13.10<br /> <br /> Checking E-Mail Addresses, Version 2.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Checking an email address - Version 02</title> <script type=”text/javascript” language=”javascript”> <!-- //<br /> <br /> Regular Expressions Make It Easier<br /> <br /> LISTING 13.10<br /> <br /> 435<br /> <br /> continued<br /> <br /> function IsMatchingAddress(str){ var myRegExp = /[a-z0-9-.]{1,30}@[a-z0-9-]{1,65}.(com|net|org|info|biz|([az]{2,3}.[a-z]{2}))/ ; return myRegExp.test(str) } // end function IsMatchingAddress() function TestGuess(){ var EmailAddr = new String(); entry = prompt(“Enter an email address here”, “Please type it carefully”); EmailAddr = entry; EmailAddr = EmailAddr.toLowerCase(); var validLength = (EmailAddr.length>=6); if (IsMatchingAddress(EmailAddr) && (validLength)){ alert(“You entered a valid email address: “ + EmailAddr); } // end if else if (!validLength){ alert(EmailAddr.length); alert(“Sorry you entered an email address of the wrong length. Try again.”); } else { alert(“Sorry “ + EmailAddr + “ isn’t a valid email address.”); } // end else } // --> </script> </head> <body> <h3>This page allows you to enter and check an email address such as consultancy@xmml.com,</h3> <h3>consultancy@xmml.co.uk or consultancy@xmml.org.uk</h3> <form> <button type=”Button” onclick=”TestGuess()”> Click here to enter email address</button> </form> </body> </html><br /> <br /> Tip<br /> <br /> The listings given for checking e-mail addresses only check to determine whether the structure of the string entered corresponds to an e-mail address. To ensure that spelling or typing errors such as, conslutancy@xmml.com, are caught, consider asking the user to enter an e-mail address twice. Also think about building in a further check, by sending a message to the e-mail address asking for confirmation, so that a malicious visitor to your site doesn’t cause problems for the genuine owner of an e-mail address.<br /> <br /> 13<br /> <br /> 436<br /> <br /> Day 13<br /> <br /> Now that you have an understanding of regular expressions up to this point, let’s take a step to one side and look a little more formally at regular expressions.<br /> <br /> Regular Expressions Overview The RegExp object is a Core JavaScript object. There is one RegExp object per Window object. Regular expressions were introduced to JavaScript in the version 4 browsers—that is, with JavaScript 1.2. If a significant percentage of the visitors to your site use pre-version 4 browsers, you will need to avoid using regular expressions at all in your JavaScript code, or use them only within a suitable routine to provide conditional processing— depending on whether or not a user’s browser supported regular expressions. Although the scenario of pre-version 4 browser use is becoming less and less important, you would likely need to apply regular expressions or some equivalent string-validation on the server side, thereby slowing down validation of user entered text.<br /> <br /> Defining Regular Expressions As for all the other Core JavaScript objects that we have met, regular expressions can be created in one of two ways. You can use either literal notation (as we have done up to this point) or the RegExp object as a constructor function. Both of these methods create a full object, but there are some differences between them.<br /> <br /> Creating a Regular Expression Using Literals The way we have constructed a regular expression thus far in this chapter is to use literal notation. When creating a regular expression literal we have to mark it out just as we marked out a string literal by surrounding it with quote marks. In the case of regular expression literals, we use a different character to mark out our literals. We use the forward slash character to indicate the start of the regular expression and to mark its end. Here is an example: var myRegExp = /Sams/;<br /> <br /> As you can see, this is fairly simple. We have created a new regular expression object (using literal notation) and assigned it to the variable myRegExp. This regular expression will match the string “Sams” in any string it is applied to. Of course this isn’t any different than what we could achieve by using the indexOf() method of the String object and giving it the parameter “Sams”.<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 437<br /> <br /> As you have seen, the real power of regular expressions is in matching patterns as opposed to exact strings. In order to match certain characters, you need to have a means of representing these characters. For example, you may need to know whether a string contains at least one number, but not be concerned as to which number it is. You could achieve this by doing 10 searches for each numerical digit using the String object’s indexOf() method, but this is very inefficient when you could just do it in one search using regular expressions. To match certain characters you need to use special characters. These special characters are simply normal characters that have been escaped using a backslash. For example \d is the special character that matches any of the 10 numeric digits, 0 to 9 inclusive. Therefore to create a regular expression that could be used to search for a numerical digit you would write the following: var myRegExp = /\d/;<br /> <br /> Alternatively, you can use the syntax you saw earlier: var myRegExp = /[0-9]/ ;<br /> <br /> As you can see when you use an escaped character, this is beginning to show signs of the possibility of regular expression becoming very cryptic. The nature of having characters that represent a group of characters will inevitably mean that regular expressions will not be immediately readable. After we look at creating a regular expression using a constructor function, we will move on and take a more detailed look at the special characters available to us when using regular expressions in JavaScript.<br /> <br /> Creating Regular Expressions Using a Constructor The other option that we can use to create a new regular expression is shown here as follows: var myRegExp = new RegExp(“[0-9]”);<br /> <br /> You may have noticed that instead of using a pair of forward slashes as delimiters (as we did when using literal notation), we used a pair of quote marks when using the RegExp() constructor. You might well suggest that what we have, delimited by quote marks, is a string. If so, you would be correct.<br /> <br /> Note<br /> <br /> Remember that the delimiter is / for literal regular expression syntax, and that the constructor syntax uses the “ character as the delimiter.<br /> <br /> 13<br /> <br /> 438<br /> <br /> Day 13<br /> <br /> When we use the RegExp() constructor we are actually storing a string, rather than a regular expression. This means that when we want to use that string as a regular expression then the JavaScript interpreter has to convert the content of the string to a regular expression. This form, which is already in place when we used the literal notation, allows the use of the regular expression by the JavaScript interpreter. The time to compile the string to the internal format for the regular expression is not long, but will make a slightly less efficient computation. However, using the RegExp() constructor allows you to more easily create or define a new regular expression from a string variable. You can use the RegExp() constructor, for example, to create a test bed to check how well you have grasped regular expression syntax. Listing 13.11 shows a simple test bed that allows you to easily create and test regular expression patterns. Notice that it is a pattern without the / delimiters which is needed.<br /> <br /> INPUT<br /> <br /> LISTING 13.11 Creating and Testing Regular Expressions (RegExpTester.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Regular Expression Tester</title> <script type=”text/javascript” language=”javascript”> <!-- // var myRegExp = “”; function SetFocus(){ document.SimpleForm.FirstInput.focus(); } // end function SetFocus() function SubmitPattern(){ var pattern = document.SimpleForm.FirstInput.value; if (pattern.length==0){ alert(“You made an invalid entry. Please start again.”); document.SimpleForm.reset(); SetFocus(); } // end if else { // pattern is at least one character long myRegExp = new RegExp(pattern); alert(“Current pattern is “ + myRegExp); } // end else } // end function SubmitPattern()<br /> <br /> Regular Expressions Make It Easier<br /> <br /> LISTING 13.11<br /> <br /> 439<br /> <br /> continued<br /> <br /> function TestString(){ testString = document.SimpleForm.TestInput.value; if (IsMatchingString(testString)){ alert(“The test string: “ +testString + “\nmatched the current pattern: “ + myRegExp); } // end if else{ alert(“The test string: “ +testString + “\ndid not match the current pattern: “ + myRegExp); } // end else } // end function TestString() function ClearAndSet(){ document.SimpleForm.reset(); SetFocus(); MaxNum = 0; } function IsMatchingString(str){ return myRegExp.test(str) } // end function IsMatchingString() // --> </script> </head> <body onload=”SetFocus()”> <h3>This page allows you to create and test regular expression patterns</h3> <h3><b>Be careful! - There is no logic to test string length.</b></h3> <form name=”SimpleForm”> <table> <tr> <td width=”30%” align=”right”>Enter a pattern:</td> <td><input name=”FirstInput” type=”text”></td> </tr> <tr> <td width=”25%” align=”right”> </td> <td><button type=”Button” onclick=”SubmitPattern()”> Submit new pattern</button> </td> </tr> <tr> <td width=”30%” align=”right”>Enter a string to match the pattern:</td> <td><input name=”TestInput” type=”text”></td> </tr> <tr> <td width=”25%” align=”right”><button type=”Button” onclick=”ClearAndSet()”> Clear and Start again</button></td> <td><button type=”Button” onclick=”TestString()”> Click to test the string you entered</button></td> </tr><br /> <br /> 13<br /> <br /> 440<br /> <br /> Day 13<br /> <br /> LISTING 13.11<br /> <br /> continued<br /> <br /> </table> </form> </body> </html><br /> <br /> This page allowed you to enter a regular expression pattern as a string. When the “Submit new pattern” button is clicked an alert box shows the user the pattern that has been created and which is available for testing. The user then has the option of entering test strings in the second input box and submitting them for testing against the regular expression when the “Click to test the string you entered” button is clicked.<br /> <br /> ANALYSIS<br /> <br /> Note<br /> <br /> When using Listing 13.11 to test regular expression patterns you need to be aware that the special characters (described in the next section) need to be entered double escaped. Thus, for example, to achieve “\b” in the regular expression created from the string, you would enter “\\b” in the form text field.<br /> <br /> Special Characters There are certain characters that we can’t simply write into our regular expressions in order to find a match for them. These characters are the regular expression special characters. And, because they have special meaning, simply writing them in as characters and expecting a match will produce unexpected results or an error. For example, we can’t just place a forward slash into a regular expression literal and expect a match on the forward slash character. The forward slash denotes the beginning and end of a regular expression. So if we want to find a match for the string “Dear Sir/Madam” we could not just write the following: var myRegExp = /Dear Sir/Madam/;<br /> <br /> JavaScript would get to the forward slash after “Sir” and think that the forward slash indicated the end of the regular expression. When the JavaScript interpreter processed the rest of the line and found the string “Madam”, it would attempt to process it as a piece of JavaScript syntax and an error would result. The correct syntax is to use a literal forward slash character within a regular expression to precede the forward slash, which is intended to be interpreted as a literal, with a backslash, like so: var myRegExp = /Dear Sir\/Madam/;<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 441<br /> <br /> There are several other special characters used in regular expressions that we can’t just write into a regular expression. These include the following: \ ^ $ * + ? . ( ) { } [ ] |<br /> <br /> All these characters have special meaning when used within regular expressions and will not be literally matched. You have seen, for example, how the curly braces are used to enclose the number of permitted occurrences of a character literal or a character class. If you want to actually match special characters literally, then you will need to remove their special meaning. You can do this by using another special character, the backslash character, to escape them. As you will see, the backslash character is a very important character in regular expressions. To create a regular expression that matches the string “Enter (Yes/No)” you would write the following: var myRegExp = /Enter \(Yes\/No\)/;<br /> <br /> By using the backslash character to escape the forward slash in the string “Yes/No”, you remove the forward slash’s special meaning and so the JavaScript interpreter knows the forward slash is part of the pattern and not the end of the regular expression. The backslash itself isn’t part of the string to be matched. It only has influence on the character that follows it, as in the example above, to remove special meaning. Similarly the backslash which precedes the opening parenthesis, “(“, and the closing parenthesis, “)”, indicate that the character is to be interpreted literally, rather than used to group a subexpression within a regular expression. The backslash can be used in the same way with all the other special characters in the list shown earlier. If you want to match them literally as opposed to using them with their special meaning, then you would need to escape them with the backslash character. One thing that may be confusing about matching characters that have special meaning is in matching the backslash itself. In the above examples, we have used the special meaning of the backslash character to remove the special meaning of other special characters. But how do we remove the special meaning from the backslash character itself? You simply use another backslash character. So if you (for some unknown reason) wanted to match the string c:\My Writing\TY JavaScript<br /> <br /> then you could create a regular expression like this: var myRegExp = /c:\\My Writing\\TY JavaScript/;<br /> <br /> Each time you see \\ within the regular expression indicates a literal backslash character.<br /> <br /> 13<br /> <br /> 442<br /> <br /> Day 13<br /> <br /> Earlier in the chapter you learned about special characters such as the curly braces. Shortly, we will examine what the other special characters are used for, but for now let’s look more closely at how to use the backslash character. The backslash character is key to working with regular expressions. Not only does it remove special meaning from special characters so you can actually include them in the pattern to be matched, but it also adds special meaning to non-special characters. At first this may seem confusing, but you have already seen an example of this behavior in use. We mentioned previously that you can match any of the 10 numerical digits by using the following regular expression: var myRegExp = /\d/;<br /> <br /> If the regular expression had simply been, var myRegExp = /d/;<br /> <br /> then it would have matched the character “d”. But by escaping the d we give it special meaning. In this case the escaped “d” now represents any digit from 0 through 9. Table 13.1 shows a list of the special characters, including those that will be of most use to you that can be created by escaping a letter. TABLE 13.1<br /> <br /> The Special Characters in JavaScript Regular Expressions<br /> <br /> Escaped Character<br /> <br /> Character(s) Matched<br /> <br /> ^<br /> <br /> Beginning of input<br /> <br /> $<br /> <br /> End of input<br /> <br /> .<br /> <br /> Any character but the new line character (\n)<br /> <br /> \b<br /> <br /> Any word boundary character<br /> <br /> \B<br /> <br /> Any character that is not a word boundary character<br /> <br /> \d<br /> <br /> Any numeric digit character<br /> <br /> \D<br /> <br /> Any character that is not a numeric digit character<br /> <br /> \n<br /> <br /> A new line (line feed) character<br /> <br /> \r<br /> <br /> A carriage return character<br /> <br /> \s<br /> <br /> Any whitespace character<br /> <br /> \S<br /> <br /> Any character that is not a whitespace character<br /> <br /> \t<br /> <br /> A tab character<br /> <br /> \w<br /> <br /> Any alphanumeric character (includes the underscore character)<br /> <br /> \W<br /> <br /> Any character that is not an alphanumeric character (excludes the underscore character)<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 443<br /> <br /> Notice that \n matches the new line character. You have used this syntax in connection with alert boxes in earlier chapters. Most of the special characters are created by escaping a letter that gives some indication as to which character(s) it will match. For example, \s is used for the special character that matches whitespace. Hopefully this will help to make it easier to remember which letter should be escaped to create the special character you need. You may have noticed another implicit rule in the letters used to create special characters. If the letter is lowercase, then it matches the characters indicated by the escaped character, but if it is uppercase then it matches any characters except the characters indicated by the letter. Again this should make it slightly easier to remember which letter is used to create which special character. Some characters matched by some of the special characters, or the differences between some special characters, may not be immediately apparent. So let’s clarify their meanings.<br /> <br /> Whitespace Versus Word Boundary The distinction between the word boundary special character \b and the whitespace special character \s is one of those that may initially be unclear. However the key difference between the two can be made clear through an example. For example, you may want to check for a word in a string, such as “man”—a sub-string, but an actual individual word, “man”; but not to include words such as “manly” or “manual”. This means that if you were looking for “fun” you wouldn’t want a match if the word “funny” was in the string. To do this you need to wrap special characters around the word “man” or “fun” to indicate that you are looking for the exact word and not just the pattern “man” or “fun”. The following code does this by making use of the whitespace special character: var sentence = “What fun regular expressions are.”; var myWord = /\sfun\s/; var wordFound = myWord.test(sentence); alert(wordFound);<br /> <br /> In the example above the regular expression will match any string that starts with a whitespace character followed by the letters “f”, “u” and “n”, and then another whitespace character. Because that string is indeed within the string contained in the variable called sentence the test will return true. But there are situations when using whitespace to mark out a word will fail to return the correct result. Let’s change the string slightly and see what happens:<br /> <br /> 13<br /> <br /> 444<br /> <br /> Day 13<br /> <br /> var sentence = “What ‘fun’ regular expressions are.”; var myWord = /\sfun\s/; var wordFound = myWord.test(sentence); alert(wordFound);<br /> <br /> This time the test returns the value false. Although the word “fun” is in the string, because we specified that a match would be surrounded by whitespace characters, the regular expression didn’t match it. The apostrophe (or single quote) is not a whitespace character. The solution is to use the regular-expression tool specifically designed for the job—the word-boundary special character. The word-boundary character will match all whitespace characters but in addition to this it will match punctuation marks such as commas, full stops, quotation marks, or any other characters that can come before or after a word in written English. By replacing the whitespace special characters with word-boundary special characters in the example above, we will obtain the result we were looking for: var sentence = “What ‘fun’ regular expressions are.”; var myWord = /\bfun\b/; var wordFound = myWord.test(sentence); alert(wordFound);<br /> <br /> The above code would return true, since the regular expression recognizes the word boundary expressed by the apostrophe (single quote) characters that delimit “fun”. Of course, at times the whitespace special character will be what you are looking for. But you should be careful that it is specifically whitespace that you want to match. If searching for a word, then the word-boundary character is the one to use if you don’t want to miss words that are contained, for example, in quote marks.<br /> <br /> Matching Any Alphanumeric Character The alphanumeric special character, \w, will match both uppercase and lowercase letters as well as numbers and the underscore character. Conversely, the special character \W will match any character except an alphabetical character or numerical character.<br /> <br /> Quantifiers The special characters in the previous section simply match one character. We could get around this by writing multiple character classes, but as you saw in earlier examples, we can use syntax such as curly braces, {1,2} for example, to indicate the lower and upper permitted cardinality. However, in regular expressions certain common occurrences have a more abbreviated syntax. If a character is optional, you can indicate that situation using the ? cardinality operator. So if you have part numbers that are of the form of two uppercase alphabetic<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 445<br /> <br /> characters, two numeric digits, and an optional alphabetic character, you can express that using the following code: var myRegExp = /[A-Z]{2}\d[A-Z]?/<br /> <br /> Notice that following the second character class [A-Z], we used the ? character to indicate that that final character is optional—that is, it may occur 0 or 1 times. Alternatively, but slightly less compactly, we could have expressed the same pattern using the curly brace syntax, like so: var myRegExp = /[A-Z]{2}\d[A-Z]{0,1}/<br /> <br /> If we wanted to express the idea that the final character was necessary but could occur multiple times, then we would use the + character: var myRegExp = /[A-Z]{2}\d[A-Z]+/<br /> <br /> An alternate way of expressing that idea using curly braces is shown here: var myRegExp = /[A-Z]{2}\d[A-Z]{1,}/<br /> <br /> The syntax {1,} indicates that the minimum occurrence is one time but that the upper limit for occurrence is undefined—that is, the character class must occur at least once but is permitted to occur an unlimited number of times. Similarly if we wanted to indicate the final character is optional but it is also permitted to occur an unlimited number of times, then we would use the * character: var myRegExp = /[A-Z]{2}\d[A-Z]*/<br /> <br /> The equivalent meaning expressed using the curly braces syntax is shown here: var myRegExp = /[A-Z]{2}\d[A-Z]{0,}/<br /> <br /> In other words the minimum permitted occurrence is zero times and the maximum is unbounded. The cardinality syntax in regular expressions is summarized in Table 13.2. TABLE 13.2<br /> <br /> Cardinality Syntax in JavaScript Regular Expressions<br /> <br /> Quantifier<br /> <br /> Matched Preceding Character<br /> <br /> *<br /> <br /> Zero or more times<br /> <br /> ?<br /> <br /> Zero or one times<br /> <br /> +<br /> <br /> One or more times<br /> <br /> {n}<br /> <br /> Exactly n times<br /> <br /> {n,}<br /> <br /> n or more times<br /> <br /> {n,m}<br /> <br /> Between n and m times (inclusive)<br /> <br /> 13<br /> <br /> 446<br /> <br /> Day 13<br /> <br /> The Methods of the RegExp Object Like many other JavaScript objects, the RegExp object has several methods. The methods of the RegExp object are listed here: •<br /> <br /> compile()<br /> <br /> •<br /> <br /> exec()<br /> <br /> •<br /> <br /> test()<br /> <br /> •<br /> <br /> toString()<br /> <br /> •<br /> <br /> valueOf()<br /> <br /> •<br /> <br /> watch()<br /> <br /> •<br /> <br /> unwatch()<br /> <br /> The simplest of the methods used to search for a string is the test() method, which tests for a string or substring which matches the regular expression pattern. It will return true if it finds a substring that matches the pattern described by the regular expression used, or false if it doesn’t. Regular expression methods are applied to the regular expression containing the pattern you wish to test for, passing the string to be examined as a parameter. For example, to test for numbers in a string called myAge you might use the following lines of code: var myAge = “I am 21 years old”; var digits = /\d/; var digitsPresent = digits.test(myAge); alert(“There are “ + (digitsPresent? “”: “no”) + “ digits.”);<br /> <br /> In this example the value returned is true. The regular expression finds the number 2 in “21” and stops there to return true.<br /> <br /> Using the exec() Method The exec() method provides more information about a match found in a string. It provides this information in the form of an Array object that it returns when evaluated. Therefore to capture this information you need to use the exec() method to the right of an assignment operator and assign the returned array to a variable, like so: var results = RegExp.exec(str);<br /> <br /> As well as having data filled out in some of its elements, the array that is returned has two properties that are of interest to us. These are the index and input properties. The input property is simply the string that was sent to the exec() method as its parameter to search through. The index property, on the other hand, contains a number that is the index of the first character of the match in the input.<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 447<br /> <br /> The element that is of most interest to us is the first element of the array. It contains the substring that the exec() method found that matched the pattern in the regular expression it was applied to. For example, as shown in the following text: var caps = /[A-Z]/; var str = “a sentence With some Capital letters” var result = caps.exec(str); alert(result[0]); // alerts “W”<br /> <br /> The contents of the first element of the array that was returned from the exec() method was the string it matched. In this case that was simply the uppercase letter W. The contents of this first element are identical to the lastMatch property of the core RegExp object. The benefit of this is that, as long as we don’t intentionally overwrite the contents of the variable result, the array will maintain the results of an individual search that returned a match even after doing multiple searches using regular expressions. However each new search will assign new values to the RegExp object’s properties. The other elements of the array that are returned are also copies of the values assigned to the array. If no match is found using the exec() method, then the value null is returned in place of an array. This enables you to test whether or not you should attempt to carry out any processing on possible results that may have been returned.<br /> <br /> Using the test() Method The RegExp object’s test()method’s main function is to check for a match in a string. Each time the test() method finds a match it sets the lastIndex property of the regular expression used to the string index of the last character in the match. The lastIndex property tells the test() method from where to start checking. When using the test() method repeatedly on the same string, this allows a later test of the regular expression to start where the previous one left off.<br /> <br /> 13 Caution<br /> <br /> Always set the lastIndex property of a regular expression to 0 after each test if it is not going to be used again on the same string. If the lastIndex property is not set to zero, then a search for a match may start at an unpredictable place part way through a string.<br /> <br /> 448<br /> <br /> Day 13<br /> <br /> The Properties of the RegExp Object The RegExp object has the following properties that are listed here. Only some of these will be discussed. •<br /> <br /> $1-$9<br /> <br /> •<br /> <br /> $*, $&, $_, $+<br /> <br /> •<br /> <br /> constructor<br /> <br /> •<br /> <br /> global<br /> <br /> •<br /> <br /> ignoreCase<br /> <br /> •<br /> <br /> input<br /> <br /> •<br /> <br /> lastIndex<br /> <br /> •<br /> <br /> lastMatch<br /> <br /> •<br /> <br /> lastParen<br /> <br /> •<br /> <br /> leftContext<br /> <br /> •<br /> <br /> multiline<br /> <br /> •<br /> <br /> prototype<br /> <br /> •<br /> <br /> rightContext<br /> <br /> •<br /> <br /> source<br /> <br /> Scoping a Regular Expression Using the global Property The global property specifies whether or not a search should be made for all possible matches of a pattern. The global property can be set or tested directly. Alternatively the literal regular expression syntax allows a succinct way to express it, by adding a single character “g” after the closing forward slash of a regular expression, like so: var myString = “My man created a manual manually”; var myRegExp = /man/ ;<br /> <br /> It will search for the pattern man and stop when the first occurrence is found. However, by either setting the global property explicitly or adding a g flag to the literal syntax the full string is searched, rather than stopping after finding the first match. var myString = “My man created a manual manually”; var myRegExp = /man/g ;<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 449<br /> <br /> Making a Regular Expression Case Insensitive Using the ignoreCase Property The ignoreCase property can be set explicitly, as it is here: var myRegExp = new RegExp(); myRegExp.ignoreCase = true; myRegExp.test(“i doN’t CAre what case is”);<br /> <br /> The same effect can be achieved using the literal regular expression syntax using the i flag, following the closing forward slash of the literal syntax: var myRegExp = /i doN’t CAre what case is/i ;<br /> <br /> There is much more to regular expressions than we have been able to introduce you to in this chapter. We hope that you have been able to see how powerful regular expressions can be and will want to explore further the power they provide.<br /> <br /> Summary In this chapter, we looked at some practical problems of string validation that you might encounter in a production Web site. We also explored step-by-step how regular expressions provide the techniques to allow you to validate the structure of strings entered to represent textual or numerical data. If you are able to express in plain English the pattern you want to represent, then you have already learned many of the parts of regular expression syntax that represent them and allow you to apply powerful validation techniques in your own Web sites.<br /> <br /> Workshop In this workshop we will review what you have learned about regular expressions in this chapter.<br /> <br /> Q&A Q. If some data is entered and tested using a regular expression, can I be sure that the data can be processed further? A. No, you cannot be certain about that. For example, if a currency amount is entered a regular expression can check if it is a valid structure but that doesn’t mean you can use the string (which represents a number) in numerical calculations. You may need to use the Number() method to ensure that the value is used as a number.<br /> <br /> 13<br /> <br /> 450<br /> <br /> Day 13<br /> <br /> Q. If I use the correct regular expression when validating data, do I, or a colleague, need to carry out validation on the server side? A. Someone still needs to make checks on the server side. For example, when validating a form with user contact information, indicate to the users that they will receive an e-mail requesting confirmation in a few minutes, and if they don’t receive it then they should re-enter their contact details. Additionally, you may need to restructure data before storing it in a database. If you have a field for a U.S. SSN and allow entry to be in the formats 123-4567-89 or 123456789 then you will need to convert entries to a single format if you want to achieve efficient sorts or searches within the data.<br /> <br /> Quiz 1. What flag do you use to indicate that a regular expression pattern is case insensitive? 2. Which special character do you use in a regular expression pattern to represent numeric digits? 3. Give two ways in which you can indicate that a character is optional, but if it occurs it can occur only once. 4. What character indicates that a character or character class must be present, but can occur more than once?<br /> <br /> Quiz Answers 1. Adding an i after the closing forward slash indicates the pattern is to be applied case insensitive. 2. You can use the \d special character to indicate numeric digits. Alternatively you can use the [0-9] character class. 3. The ? cardinality character indicates that a character is optional but, if it occurs, it can occur only once. We can also indicate the same by writing the following: {0,1}. 4. The + character indicates that a character or character class occurs one or more times.<br /> <br /> Exercises 1. Adapt the Regular Expression Tester (RegExpTester.htm) to allow a string length to be entered and to add the length to the testing of test strings which the user has entered.<br /> <br /> Regular Expressions Make It Easier<br /> <br /> 451<br /> <br /> 2. Create a program, using regular expression patterns, to test a United Kingdom Social Security Number, which takes the form “AB 12 34 56 C.” The first two characters and the final character may be any uppercase alphabetic characters. 3. Create a program to test whether a string entered is a valid U.S. telephone number. Remember that the opening and closing parentheses have a special purpose in regular expressions.<br /> <br /> 13<br /> <br /> WEEK 2<br /> <br /> DAY<br /> <br /> 14<br /> <br /> Advanced Array Management In Chapter 5, “An Introduction to Arrays,” you were introduced to JavaScript arrays and some of the ways in which you can use them. In this chapter, we will build on what you learned earlier, examine more fully the methods of the Array object, and go on to look at how you can use arrays to represent multidimensional data structures. This chapter will teach you • How to use more of the Array object’s methods • How to simulate a multidimensional array<br /> <br /> The Array Object’s Methods In Chapter 5 we looked at several of the Array object’s methods, but didn’t look at all of them. In this chapter we will complete our study of those methods, enabling us to access the full power of arrays for storing and managing our data.<br /> <br /> 454<br /> <br /> Day 14<br /> <br /> Here is a list of the methods of the Array object: •<br /> <br /> concat()<br /> <br /> •<br /> <br /> join()<br /> <br /> •<br /> <br /> pop()<br /> <br /> •<br /> <br /> push()<br /> <br /> •<br /> <br /> reverse()<br /> <br /> •<br /> <br /> slice()<br /> <br /> •<br /> <br /> shift()<br /> <br /> •<br /> <br /> sort()<br /> <br /> •<br /> <br /> splice()<br /> <br /> •<br /> <br /> toLocaleString()<br /> <br /> •<br /> <br /> toSource()<br /> <br /> •<br /> <br /> toString()<br /> <br /> •<br /> <br /> unshift()<br /> <br /> •<br /> <br /> valueOf()<br /> <br /> In Chapter 5, we looked at all of the methods except the concat(), slice(), sort(), and splice() methods. However these are the Array object’s most powerful methods. We need to understand how to use them to unleash the potential of JavaScript for structuring and manipulating our data using arrays.<br /> <br /> The concat() Method The concat() method enables you to add one array to the end of another array to create a third and longer array. The name of the concat() method comes from the term concatenation, which you may remember from our discussion of the plus sign acting as the concatenation operator for adding one string to the end of another. Although the concat() method isn’t quite as simple to use as the concatenation operator, it’s still relatively easy. The concat() method is applied to an array using the dot notation, and the name of the array to be concatenated to it is written into the parentheses as a parameter. Listing 14.1 shows a simple example of how to use the concat() method to combine two arrays.<br /> <br /> Advanced Array Management<br /> <br /> INPUT<br /> <br /> 455<br /> <br /> LISTING 14.1<br /> <br /> Using the concat() Method to Combine Two Arrays (ConcatArrays01.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using concat() method to combine two arrays</title> <script type=”text/javascript” language=”javascript”> <!-- // function ConcatArrays(){ var firstArray = new Array(1,2,3); var secondArray = new Array(4,5,6); var combinedArray = firstArray.concat(secondArray); var fa = firstArray.join(“, “); var sa = secondArray.join(“, “); var ca = combinedArray.join(“, “); document.write(“<p>The first array contains: <b>” + fa + “ </b></p>”); document.write(“The second array contains: <b>” + sa + “ </b></p>”); document.write(“The combined array contains: <b>” + ca + “ </b></p>”); document.write(“The combined array length is: <b>” + combinedArray.length + “ </b></p>”); } // --> </script> </head> <body onload=”ConcatArrays()”> </body> </html><br /> <br /> The secondArray elements have been added to the end of the firstArray elements to create the new array called combinedArray. Note that the process of concatenating two arrays creates a new array. The original two arrays from which it was made are unaffected. They both remain intact and separate. We have used the join() method of the Array object to display the elements of the first, second, and combined arrays. You may remember that the join() method allows you to display the elements of an array with a separator of your choosing.<br /> <br /> ANALYSIS<br /> <br /> Figure 14.1 shows the output from Listing 14.1. Notice particularly the content of the combined array and the sequence of elements.<br /> <br /> 14<br /> <br /> 456<br /> <br /> Day 14<br /> <br /> OUTPUT FIGURE 14.1 The output after concatenating the secondArray to the firstArray.<br /> <br /> When concatenating two arrays it is essential to make sure that you understand the order of elements in the new array. Listing 14.2 shows how you can concatenate firstArray to secondArray. In that situation, as you can see in Figure 14.2, the ordering of elements in the combined array is different. If you had assumed the ordering which resulted from Listing 14.1 but used the approach in Listing 14.2, you likely would have unexpected results when you processed one or more array elements.<br /> <br /> INPUT<br /> <br /> LISTING 14.2 The concat() Method Used in the Opposite Order (ConcatArrays02.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using concat() method to combine two arrays</title> <script type=”text/javascript” language=”javascript”> <!-- // function ConcatArrays(){ var firstArray = new Array(1,2,3); var secondArray = new Array(4,5,6); var combinedArray = secondArray.concat(firstArray); var fa = firstArray.join(“, “); var sa = secondArray.join(“, “); var ca = combinedArray.join(“, “); document.write(“<p>The first array contains: <b>” + fa + “ </b></p>”); document.write(“The second array contains: <b>” + sa + “ </b></p>”); document.write(“The combined array contains: <b>” + ca + “ </b></p>”); document.write(“The combined array length is: <b>” + combinedArray.length + “ </b></p>”); }<br /> <br /> Advanced Array Management<br /> <br /> LISTING 14.2<br /> <br /> 457<br /> <br /> continued<br /> <br /> // --> </script> </head> <body onload=”ConcatArrays()”> </body> </html><br /> <br /> OUTPUT FIGURE 14.2 The output after concatenating the firstArray to the secondArray.<br /> <br /> Making Copies of an Array At this point let’s go back and consider the difference between a variable name actually containing an array (which it doesn’t) and containing a reference to an array (which it does). We mentioned in Chapter 5 that arrays don’t actually have their own names. They are only accessed by data containers (usually a variable), which hold a reference to the array. The significance of this may not be immediately obvious. You can use the variable name just as if it was the name of the array. However, if you try to create a copy of an array and don’t understand this topic, then you can run into some difficulties, which can waste a lot of time. Before you consider making copies of arrays, let’s first review how we make a copy of a variable. There is a major difference between the way you create a copy of a variable and the way you create a copy of an array. For variables it is very simple: You simply declare a variable name for the copy and assign the value of the original to the new variable as shown below: var myVar = “some value”; var myCopy = myVar;<br /> <br /> 14<br /> <br /> 458<br /> <br /> Day 14<br /> <br /> In this way, you can perform any number of operations on the original variable myVar and then, if needed, revert back to its original value as stored in the variable myCopy. Unfortunately when you need to create a copy of an array things are not quite so simple. Let’s look at an example (Listing 14.3) to see what would happen if we were to attempt it in the same way that we copied the variable myVar. LISTING 14.3<br /> <br /> INPUT<br /> <br /> The Seemingly Successful Copying of an Array (CopyArray01.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Seeming success in copying an array</title> <script type=”text/javascript” language=”javascript”> <!-- // function CopyArray(){ var myArray = new Array(1,2,3); var myCopy = myArray; var ma = myArray.join(“, “); var mc = myCopy.join(“, “); document.write(“<p>The original array <u><code>myArray</code></u> contains: “ + ma + “ </p>”); document.write(“<p>The copied array <u><code>myCopy</code></u> contains: “ + mc + “ </p>”); } // --> </script> </head> <body onload=”CopyArray()”> </body> </html><br /> <br /> Figure 14.3 shows the result of the (seeming) array copy. Our attempt at copying an array seems to have worked. However, let’s make some changes to the original array myArray, and then see if we can revert to its original values using the copy of the array, myCopy. You can test this with Listing 14.4.<br /> <br /> Advanced Array Management<br /> <br /> 459<br /> <br /> OUTPUT FIGURE 14.3 The seemingly successful result of an array copy.<br /> <br /> LISTING 14.4<br /> <br /> INPUT<br /> <br /> Copying an Array and Then Changing the Original (CopyArray02.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Seeming success in copying an array</title> <script type=”text/javascript” language=”javascript”> <!-- // function CopyArray(){ var myArray = new Array(1,2,3); var myCopy = myArray; myArray[0] = 10; myArray[1] = 20; myArray[2] = 30; var ma = myArray.join(“, “); var mc = myCopy.join(“, “); document.write(“<p>The original array <u><code>myArray</code></u> contains: “ + ma + “ </p>”); document.write(“<p>The copied array <u><code>myCopy</code></u> contains: “ + mc + “ </p>”); } // --> </script> </head> <body onload=”CopyArray()”> </body> </html><br /> <br /> 14<br /> <br /> 460<br /> <br /> Day 14<br /> <br /> If you run Listing 14.4, you will see the result shown in Figure 14.4. Both the original array and the copy of that array return the same values for the array elements.<br /> <br /> OUTPUT FIGURE 14.4 Copying and changing an array changes its copy too.<br /> <br /> The changes we made to the original array myArray also seem to have affected the copy myCopy! At first sight that may seem bizarre, and it can cause a great deal of frustration if you don’t know what’s going on. The reason that the changes we made to the original array myArray were also reflected in the copy myCopy, is that myCopy wasn’t actually a copy of the array at all. To understand this we need to come back to the topic of references.<br /> <br /> ANALYSIS<br /> <br /> The variable myArray contained a reference to a part of the computer’s memory where the array originally created was stored. The consequence of this is that when we tried to copy the array we had created to a new variable called myCopy, myCopy was really just a copy of the reference that the variable myArray contained. This means the variable myCopy ended up pointing to exactly the same array as the variable myArray. They never were two distinct arrays—there was only one array but two separate references pointing to the same array. Whenever we create our own objects the names we give them are always variables, and as such they only contain references to the objects we create. These references are internal and we don’t get to see them, but they are real nonetheless. We can only create true copies of actual pieces of data, so normally when we want to create a copy of an object, we need to copy each property individually. Thankfully when it comes to arrays this is not the case. We can use the concat() method to save time and effort. In the previous examples of using the concat() method, the method was used to concatenate one array with another. The two arrays it was applied to were left intact and separate. The new array had to be assigned to a new variable name. To achieve this the<br /> <br /> Advanced Array Management<br /> <br /> 461<br /> <br /> method internally copied each piece of data contained in the elements of the original arrays to build up a new array. This built-in copying feature can be used to our advantage and save us from writing the code to do it. To do this, we simply need to concatenate the array to be copied with an empty array. Listing 14.5 shows how we would modify the previous example to do this.<br /> <br /> concat()<br /> <br /> LISTING 14.5<br /> <br /> INPUT<br /> <br /> Using the concat() Method of the Array Object to Create a True Copy (CopyArray03.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Actual success in copying an array using concat()</title> <script type=”text/javascript” language=”javascript”> <!-- // function CopyArray(){ var myArray = new Array(1,2,3); var emptyArray = new Array(); var myCopy = myArray.concat(emptyArray); myArray[0] = 10; myArray[1] = 20; myArray[2] = 30; var ma = myArray.join(“, “); var mc = myCopy.join(“, “); document.write(“<p>The original array <u><code>myArray</code></u> contains: “ + ma + “ </p>”); document.write(“<p>The copied array <u><code>myCopy</code></u> contains: “ + mc + “ </p>”); } // --> </script> </head> <body onload=”CopyArray()”> </body> </html><br /> <br /> When we run the code in Listing 14.5 we see that we have successfully copied the array, as you can see in Figure 14.5.<br /> <br /> 14<br /> <br /> 462<br /> <br /> Day 14<br /> <br /> OUTPUT FIGURE 14.5 A successful array copied using the concat() method.<br /> <br /> The variable myCopy has the original values of the array myArray. We have succeeded in making a true copy that we can refer back to or use as a backup—even if we have modified the original. If we use the literal notation to create the array, we can actually shorten this as shown below:<br /> <br /> INPUT<br /> <br /> var myArray = new Array(1,2,3); var emptyArray = []; var myCopy = myArray.concat(emptyArray); myArray[0] = 10; myArray[1] = 20; myArray[2] = 30;<br /> <br /> Or we can even place the array literal in the parentheses of the concat() method to make it even shorter: var myArray = new Array(1,2,3); var myCopy = myArray.concat([]); myArray[0] = 10; myArray[1] = 20; myArray[2] = 30;<br /> <br /> The only time this does not work is when one or more elements of the array being copied is a reference to another array (or other object). In this case, once again, it will be the references and not the object that will be copied. Any changes to this object will be reflected in both arrays. For the moment this is unlikely to be an issue for you as you are unlikely, initially at least, to store objects in your array elements. However, if you do start to use the multidimensional arrays that we will see later in this chapter, you may need to bear this in mind.<br /> <br /> Advanced Array Management<br /> <br /> 463<br /> <br /> The slice() Method Whenever we need to work with a limited number of consecutive elements contained within an array it is often useful to create a new array out of these elements. To create this array, we could use a loop statement to loop through the elements between two index values and copy each element to a new array, but we are saved the trouble of such custom programming by the slice() method of the Array object, which makes the task a simple one-line affair. The slice() method requires two parameters. With the first one we specify the start index of the section of the array that we want to copy; and with the second one, we specify the first index after the section to be copied. All the elements between these two indexes are copied into a new array. For example, assume we have an array of names that are ordered alphabetically and that we have found the indexes of the first and last elements with contents that begin with “B” (we can do both these things, as we will see later). To extract the names beginning with B, we could use the slice() method, as shown in Listing 14.6.<br /> <br /> INPUT<br /> <br /> LISTING 14.6<br /> <br /> Using the slice() Method to Create an Array with Selected Content (SliceArray01.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the slice() method of the Array object</title> <script type=”text/javascript” language=”javascript”> <!-- // function BNameArray(){ var allNames = new Array(“Allan”, “Bart”, “Ben”, “Bob”, “Colin”); // Process to find first and last indexes of “B’s” done here var var var var var<br /> <br /> firstB = 1; firstNonB = 4; bNames = allNames.slice(firstB, firstNonB); an = allNames.join(“, “); bn = bNames.join(“, “);<br /> <br /> document.write(“<p>The original array <u><code>allNames</code></u> contains: “ + an + “ </p>”); document.write(“<p>The array created using slice(), <u><code>bNames</code></u>, contains: “ + bn + “ </p>”); }<br /> <br /> 14<br /> <br /> 464<br /> <br /> Day 14<br /> <br /> LISTING 14.6<br /> <br /> continued<br /> <br /> // --> </script> </head> <body onload=”BNameArray()”> </body> </html><br /> <br /> Running Listing 14.6 will produce an appearance like that shown in Figure 14.6.<br /> <br /> OUTPUT FIGURE 14.6 Using the slice() method to create a new array derived from an existing array.<br /> <br /> The slice method has copied elements 1 through 3 and built up a new array, which we have assigned to the variable bNames. Note the values of the two indexes we had to specify to do this. The first index was the index of the first “B”, but the second index was the one after the index of the last element containing a name beginning with B. The index of Bob is 3, but we had to specify 4. This is a rule that is important to remember, as it is the same for all the other array methods to which we need to give index values.<br /> <br /> ANALYSIS<br /> <br /> The slice() method can only be used to select a continuous segment of an existing array. If you want to create a new array which uses more than one segment of an existing array, then you would need to concatenate the arrays created using the slice() method with the concat() method.<br /> <br /> The splice() Method As you can imagine (from Chapter 5), the four methods for removing and adding elements from the start and end of an array are often very useful, but we may need to do something slightly more sophisticated. We may want to remove or insert elements<br /> <br /> Advanced Array Management<br /> <br /> 465<br /> <br /> located part way through an array. The splice() method enables us to do this and more. Not only can we use it for the removal and insertion of elements, but it is also our means of replacing/editing multiple elements with one line of code. When we want to delete elements from an array we can use the delete statement. It is applied to one element at a time, and removes any data that is contained within that element. For example, if we subsequently try to use that element, it will have the value undefined—just as it would have if it never had been assigned a value: var x = new Array(“a”, “b”, “c”, “d”); delete x[1]; alert(x[1]); alert(x.length); alert(x);<br /> <br /> // alerts undefined // alerts 4 // alerts “a,,c,d”<br /> <br /> Note that the data contained in element 1 has been removed—that is, the value of element 1 has been set to a value of undefined. The length property of the Array object still has the value of 4, and the data in the last two elements have not been moved into earlier elements. If all you want to do is remove the data contained in a specific element, then the delete statement is the correct syntax. However sometimes when we remove an element from an array, we will want the elements that follow it to be moved back by one (assuming we deleted a single array element), thereby filling the place of the removed element and reducing the array’s length. In the example above, we might want “c” to move back to x[1], and “d” to move back to x[2], and of course the length property to be reduced by one to reflect these changes. However, the delete method was not designed to achieve this. It simply removes the data from the element to which it is applied. To carry out a more sophisticated removal of elements with the desired backshift of any following elements, we need to use the splice() method. In its simplest form the splice() method simply deletes one or more elements from inside an array, but unlike the delete method, it moves all the proceeding elements back one or more places. To achieve this we must specify 2 parameters with the splice() method. The first parameter is the index of the first element we want to remove, and the second parameter is the number of subsequent elements we want to remove. To demonstrate let’s look at the previous example modified to provide a more complete removal of the second element, as shown in Listing 14.7.<br /> <br /> 14<br /> <br /> 466<br /> <br /> Day 14<br /> <br /> INPUT<br /> <br /> LISTING 14.7<br /> <br /> Using the splice()Method of the Array Object (SpliceArray01.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the splice() method of the Array object</title> <script type=”text/javascript” language=”javascript”> <!-- // function SpliceArray(){ var x = new Array(“a”, “b”, “c”, “d”); var xj = x.join(“, “); document.write(“<p>The original array <u><code>x</code></u> contains: “ + xj + “ </p>”); document.write(“<p>The length of the original array <u><code>x</code></u> is: “ + x.length + “ </p>”); var y = x.splice(1,1); var yj = y.join(“, “); document.write(“<p>The array <u><code>y</code></u> after using splice() contains: “ + yj + “ </p>”); document.write(“<p>The length of the array <u><code>y</code></u> after splice() is: “ + y.length + “ </p>”); var xj = x.join(“, “); document.write(“<p>The original array <u><code>x</code></u> now contains: “ + xj + “ </p>”); document.write(“<p>The length of the original array <u><code>x</code></u> is now: “ + x.length + “ </p>”); } // --> </script> </head> <body onload=”SpliceArray()”> </body> </html><br /> <br /> Figure 14.7 shows the output when you run Listing 14.7.<br /> <br /> Advanced Array Management<br /> <br /> 467<br /> <br /> OUTPUT FIGURE 14.7 The array x before and after using the splice() method.<br /> <br /> As we can see, not only has the original data contained in the second element been removed, but the values c and d have been moved up to occupy elements 1 and 2, and the length property has changed to 3. Notice also that when the splice() method deleted the second element from the array x it returned it so that it was assigned to the variable y. Although this example doesn’t show it, the element isn’t just returned as a piece of data, but rather as a single element array object.<br /> <br /> ANALYSIS<br /> <br /> If we wanted to remove more elements from the array, we would just have to specify a higher number as the second parameter. For example if we want to remove the second and third elements, we would write: y = x.splice(1,2);<br /> <br /> The 2 indicates that we want to remove 2 elements, and the 1 specifies that the removal should start at element 1. As before, the removed elements would be returned by the method, this time as a two-element array. As well as using the splice() method to delete elements, we can also use it to insert new elements without removing any of the existing ones. To do this we need to specify an insertion point and the data to insert. When doing this, the first parameter of the sort() method specifies the insertion point and the second parameter must be set to zero. When the second parameter is set to zero, then the number of array elements to be deleted is zero. We will see the effect of a nonzero second parameter shortly. All the data we want to insert must then be included as extra parameters. Let’s say this time that array x is created with only two elements with the values a and d respectively. If we then want to insert the values b and c between these elements, then we could use code such as that found in Listing 14.8.<br /> <br /> 14<br /> <br /> 468<br /> <br /> Day 14<br /> <br /> INPUT<br /> <br /> LISTING 14.8<br /> <br /> Using the splice()Method to Insert Array Elements (SpliceArray02.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the splice() method to insert elements</title> <script type=”text/javascript” language=”javascript”> <!-- // function SpliceArray(){ var x = new Array(“a”,”d”, “e”, “f”); var xj = x.join(“, “); document.write(“<p>The original array <u><code>x</code></u> contains: “ + xj + “ </p>”); document.write(“<p>The length of the original array <u><code>x</code></u> is: “ + x.length + “ </p>”); x.splice(1, 0, “b”, “c”); var xj = x.join(“, “); document.write(“<p>The original array <u><code>x</code></u> now contains: “ + xj + “ </p>”); document.write(“<p>The length of the original array <u><code>x</code></u> is now: “ + x.length + “ </p>”); } // --> </script> </head> <body onload=”SpliceArray()”> </body> </html><br /> <br /> Figure 14.8.shows the result from running the code in Listing 14.8.<br /> <br /> OUTPUT FIGURE 14.8 Inserting elements into an array using the splice() method.<br /> <br /> Advanced Array Management<br /> <br /> 469<br /> <br /> ANALYSIS As you can see, the second element containing the value d has moved up to<br /> <br /> make space for the new elements b and c, and the length property has changed to reflect the addition of the new elements to the array. When applied to x the splice() method has returned an Array object.<br /> <br /> By varying the second parameter, we can do everything from a pure delete through to an exact replace to a partial or pure insertion. If we specify a number of array elements to be removed using the second parameter, then they are removed; but at the same time, we can also specify elements to insert. The number of elements deleted need not necessarily be the same as the number of new elements inserted. However, if this happens to be the case, then the result is that the elements are effectively replaced. The final use of the splice() method is, in a sense, a combination of the two uses just discussed. We can use it to replace elements within an array. Listing 14.9 shows you how.<br /> <br /> INPUT<br /> <br /> LISTING 14.9<br /> <br /> Using the splice() Method to Replace Elements Within an Array (SpliceArray03.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the splice() method to replace elements</title> <script type=”text/javascript” language=”javascript”> <!-- // function SpliceArray(){ var x = new Array(“a”,”b”, “c”, “d”); var xj = x.join(“, “); document.write(“<p>The original array <u><code>x</code></u> contains: “ + xj + “ </p>”); document.write(“<p>The length of the original array <u><code>x</code></u> is: “ + x.length + “ </p>”); x.splice(1, 2, “B”, “C”); var xj = x.join(“, “); document.write(“<p>The original array <u><code>x</code></u> now contains: “ + xj + “ </p>”); document.write(“<p>The length of the original array <u><code>x</code></u> is now: “ + x.length + “ </p>”); } // --> </script> </head> <body onload=”SpliceArray()”> </body> </html><br /> <br /> 14<br /> <br /> 470<br /> <br /> Day 14<br /> <br /> The result of running Listing 14.9 is shown in Figure 14.9.<br /> <br /> OUTPUT FIGURE 14.9 Using the splice() method to remove and insert array elements with the overall effect of replacing two elements.<br /> <br /> In this example, the first parameter of the splice() method is the number 1 and the second is the number 2. Therefore we have specified that starting at the element with index 1 two elements should be deleted. At the same time, we have also supplied two additional pieces of data to be inserted, and the insertion point for this data is also the element with the index of 1. The result is that the values b and c are removed from the second and third elements, and the values B and C are inserted at the same point. The overall effect is that the second and third elements are overwritten with the values B and C, as we can see in Figure 14.9. Note that if we had assigned to a variable y, as we did in Listing 14.7, the elements that were deleted would be returned from the splice method and stored in the variable y, as we have seen previously.<br /> <br /> ANALYSIS<br /> <br /> Normally we will want to do a delete, a replace, or an insert but, as we said earlier, it isn’t necessary that the number of elements removed be the same as the number of elements inserted. We can specify more or less of either to get a mixture of the two effects. The number of elements we specify for deletion will first be deleted, but then the number of elements we specify for insertion will be inserted at the same point. However, if you do use the splice() method in such a way, make very sure that you know the effects on the length property and the positioning of individual elements if you will need to access them later in your code.<br /> <br /> The sort() Method The sort() method is one of the most powerful of the Array object’s methods. When applied to an array in its most simple form, it rearranges the data stored in the elements of the array so that the data is ordered lexicographically. Listing 14.10 shows an example of the sort() method at work.<br /> <br /> Advanced Array Management<br /> <br /> INPUT<br /> <br /> 471<br /> <br /> LISTING 14.10<br /> <br /> Using the sort() Method to Order a Number of Strings in an Array (SortString01.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the sort() method</title> <script type=”text/javascript” language=”javascript”> <!-- // function SortElements(){ var colors = new Array(“red”,”blue”,”green”); colors.sort(); var cj = colors.join(“, “); alert(“The sorted array contains: “ + cj); } // --> </script> </head> <body onload=”SortElements()”> </body> </html><br /> <br /> Running Listing 14.10 will bring up the alert box shown in Figure 14.10.<br /> <br /> OUTPUT FIGURE 14.10 The result of an array of sorted strings.<br /> <br /> As you can see the elements of the array have been ordered as we would order them if we were to put them into alphabetical order. However, this is slightly misleading. The order in which the elements are ordered is based on the order that characters appear in the Unicode standard. Let’s look at another example to demonstrate the difference between alphabetical ordering and lexicographical ordering based on the order of the Unicode characters. This time we will include a mixture of words starting with uppercase and lowercase letters. See Listing 14.11.<br /> <br /> ANALYSIS<br /> <br /> 14<br /> <br /> 472<br /> <br /> Day 14<br /> <br /> INPUT<br /> <br /> LISTING 14.11<br /> <br /> The Effect of Case on Ordering by Using the sort() Method (SortString02.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the sort() method</title> <script type=”text/javascript” language=”javascript”> <!-- // function SortElements() { var colors = new Array(“red”,”Red”,”blue”,”Blue”,”green”,”Green”); colors.sort(); var cj = colors.join(“, “); alert(“The sorted array contains: “ + cj); } // --> </script> </head> <body onload=”SortElements()”> </body> </html><br /> <br /> The alert box displayed by Listing 14.11 is shown in Figure 14.11.<br /> <br /> OUTPUT FIGURE 14.11 When using the sort() method uppercase comes before lowercase letters.<br /> <br /> As you can see the uppercase letters have been placed before the lowercase letters. The colors beginning with uppercase letters are alphabetically ordered, and the colors beginning with a lowercase letter are alphabetically ordered, but the two groups are separate. This is because in Unicode uppercase and lowercase letters are separate, with the uppercase letters placed before the lowercase letters. Most of the time we will not want to sort our arrays like this. We will probably prefer the words in an array to be ordered alphabetically regardless of the case of the letters. Later in this section we will see how this can be done. For the moment, let’s continue with our look at the characteristics of the sort() method.<br /> <br /> ANALYSIS<br /> <br /> Advanced Array Management<br /> <br /> 473<br /> <br /> Sometimes the arrays we wish to sort will contain elements with numerical values. If we told you that numbers come before both uppercase and lowercase letters in the Unicode standard, you may think you know the order in which the elements in these types of arrays will end up. However, you may still be surprised. Let’s look at another two examples, this time with some numerical elements. See Listing 14.12.<br /> <br /> INPUT<br /> <br /> LISTING 14.12<br /> <br /> Using the sort()Method to Sort Numbers and Strings (SortStringsAndNumbers.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the sort() method on numbers and strings</title> <script type=”text/javascript” language=”javascript”> <!-- // function SortElements() { var mixture = new Array(“red”,4,”blue”,2,”green”,9); mixture.sort(); var mj = mixture.join(“, “); alert(“The sorted array contains: “ + mj); } // --> </script> </head> <body onload=”SortElements()”> </body> </html><br /> <br /> Figure 14.12 shows the results of running Listing 14.12.<br /> <br /> OUTPUT FIGURE 14.12 Using the sort() method produces numbers before letters.<br /> <br /> 14<br /> <br /> 474<br /> <br /> Day 14<br /> <br /> The numbers have been moved to the front of the array and are in the order that you most likely expected. The 2 is placed before the 4, which in turn is placed before the 9. However, as it happens, this is another deceptive example. The sort() method does not take the data type of the elements it is comparing into consideration (at least not in its unmodified form, as we are using it at the moment). The ordering is lexicographical based on the order of the characters in the Unicode standard. In the Unicode standard characters are just characters. They do not have a data type.<br /> <br /> ANALYSIS<br /> <br /> At this point you may be wondering why this matters. After all the characters seem to have been placed into the correct order. Well, let’s look at another example where the array contains some different numbers. See Listing 14.13.<br /> <br /> INPUT<br /> <br /> LISTING 14.13 A Second Example of Using sort() on Numbers and Strings (SortStringsAndNumbers02.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the sort() method on numbers and strings</title> <script type=”text/javascript” language=”javascript”> <!-- // function SortElements() { var myNums = new Array(9,4,”Bob”,11,301); myNums.sort(); var mj = myNums.join(“, “); alert(“The sorted array contains: “ + mj); } // --> </script> </head> <body onload=”SortElements()”> </body> </html><br /> <br /> Figure 14.13 shows the results of running Listing 14.13.<br /> <br /> Advanced Array Management<br /> <br /> 475<br /> <br /> OUTPUT FIGURE 14.13 Notice that the numbers have been sorted as strings by initial character, not by numerical value.<br /> <br /> At first the results of this sort may surprise you. With the 11 and 301 coming before 4 and 9, the elements of the array are most certainly not in numerical order. To understand why, let’s look more closely at what happens when we order words alphabetically.<br /> <br /> ANALYSIS<br /> <br /> For this example, let’s assume we have three words: “first,” “second,” and “third.” To put these words into alphabetical order, the first thing we do is compare the first letters of the three words. If the first letter of one alphabetically comes before the first letter of another, then it is placed first. But if the first letter of one comes after the first letter of another, then it is placed later in the order. In the case of our three words, “f” comes before “s”, so “first” is placed before “second,” and “s” comes before “t” so “second” is placed before “third.” The fact that status “first” comes before the status “second,” and status “second” comes before the status “third” has nothing to do with it. This is just coincidence. It is the first letters on which our sort is based. In the same way the fact that the numbers in the first of our two numerical examples were placed in the order they were in was also just a coincidence. When JavaScript made the comparison for the sort what it did was consider the numbers purely as characters. This time though its reference wasn’t the alphabet; it was the Unicode standard. Because the character 2 comes before the character 4 in the Unicode standard, it was placed before 4; and because the character 4 comes before the character 9, it was placed before 9. It is not because the number 2 is before the number 4 numerically, nor because the number 4 comes before the number 9 numerically. The difference between sorting numerically and sorting lexicographically using the Unicode standard is illustrated by Listing 14.13. JavaScript didn’t look at the numbers as numbers but rather it compared the first character of each number (this is what lexicographically means). The first character of 11 is 1. Because in the Unicode standard the character “1” comes before the first character of 9 (just “9”), the first character of 4 (“4”), the first character of “Bob” (“B”), and the first character of 301 (“3”), the number 11 was placed first. In the same way the first character of 301 (“3”) comes before the first character of 9, 4, and “hat,” so it was placed next. This method of ordering was continued so that 4 was placed next, then the 9, and finally the string “Bob.”<br /> <br /> 14<br /> <br /> 476<br /> <br /> Day 14<br /> <br /> Of course most of the time the order of the characters in the Unicode standard will have nothing to do with the order in which we want to sort the elements of our arrays. Most likely we will want to do either a case insensitive alphabetical sort on strings, or a numerical sort where the order of our numbers is numerical and not lexicographical. To do this we need to define our own sort criteria.<br /> <br /> Defining Our Own Sort Criteria To define our own sort criteria, we need to understand a little more about how the Array object’s sort() method works. When we sort the elements of an array using the sort() method there are two processes that go on behind the scenes. One is the comparison of the data in an array’s elements to determine the order in which they should be placed, and the other is the actual process of moving the data from element to element. It is the sort() method that moves the data from one element to another, but hidden away is a second function that does the data comparison. It is this function that tells the sort() method into which order it should move the data in an array’s elements. Because of the task it performs, this second function is often called the comparison function. The way these two functions work is that the sort() method sends the comparison function the data from two of the elements in the array it is working on, and asks the question, “should I swap the data in these elements?”. The comparison function compares the two pieces of data and returns, telling the sort() method “yes” or “no”. If the two pieces of data are the “wrong way around,” then the comparison function tells the sort() method “Yes, you should swap them.” It does this by returning the value 1, which you can think of as the numerical equivalent of the boolean value true (true to the question, “should the data be swapped?”). If the sort() method receives back the value 1 from the comparison function, then it swaps the data in the two elements and proceeds to send the comparison function another two pieces of data. Of course the pieces of data won’t always be out of order and therefore won’t always need to be swapped. When this is the case the comparison function returns a different value. When the two pieces of data it receives are in the “right” order, then the comparison function returns the value –1. This can be thought of as the numerical equivalent to the boolean value false, meaning “no”, the data in the elements should not be swapped. The third condition is of course that the two elements compared are exactly equal. In this case the elements need not be swapped, and the value returned by the comparison function is 0. This again tells the sort() method that it should not swap the data in the elements, and so is again effectively equivalent to the boolean value false.<br /> <br /> Advanced Array Management<br /> <br /> 477<br /> <br /> By making use of the comparison function, the sort() method repeatedly cycles through an array’s elements moving their data two at a time until no further changes need to be made. Once the contents of the array’s elements have been completely rearranged the sort() method stops. When we define our own sort criteria we do not need to worry about how to move the data in the elements of our arrays; the sort() method will do that for us. What we do need to concern ourselves with is the construction of a comparison function for the sort() method to use. The sort method will send this function two elements as arguments, which it should compare before returning the numerical value that tells the sort() method what it should do with the elements. To get a feel for what a comparison function should look like consider the following example. It is designed to cause the sort() method to order the elements of an array in a similar way to how it would sort them by default—lexicographically, based on the order of the characters in the Unicode standard: function defaultSort(elementX, elementY) { if (elementX < elementY) return -1; if (elementX > elementY) return 1; return 0; }<br /> <br /> To understand how this function works, recall from Chapter 2, “Working with Data,” how the comparison operators behave when used with strings. They check to see if one of their operands comes before or after their other operand based on the order of the characters in the Unicode standard! For example, x < y<br /> <br /> returns true if x comes before y, but false otherwise. This characteristic of the comparison operators makes them ideal for creating a comparison function that will order the elements of an array according to the order of the characters in the Unicode standard. In the comparison function above, called defaultSort(), we use the comparison operators as our means of checking the two elements that the function is sent. We have given the arguments of the function the names elementX and elementY to help make this clearer. The comparison function must return –1 if the first argument comes before the second by our ordering scheme. Therefore, in the first line of code in the function body we use the if statement coupled with the < operator to check for this condition and return –1 if it is true. We do not need to worry which elements the function has been sent. The sort() method will keep track of this.<br /> <br /> 14<br /> <br /> 478<br /> <br /> Day 14<br /> <br /> If, on the other hand, the first argument (elementX) comes after the data in the second (elementY) by our ordering scheme, then we need to return the value 1 so that they are swapped. This condition is checked for by the second if statement in the function body. If the data in elementX comes after the data in elementY, then 1 is returned to the sort() method. If neither of the first two return statements has been evaluated by the third line of the function body, then the data in the two elements must be identical. Therefore we include a last return statement to make the function return the value 0, indicating the data in the elements is the same. We haven’t yet seen how to tell the sort() method that it should use a comparison function, which we have defined, rather than its built-in default function. This is actually a lot easier than creating our function in the first place. You simply send the name of your comparison function to the sort() method as a parameter. For example, to tell the sort() method to use the function we have just been looking at, we would write the following: arrayName.sort(defaultSort);<br /> <br /> Note<br /> <br /> When telling the sort() method to use a function you have created, only give it the name of the function. You do not include a function call after this name (a pair of opening and closing parentheses). This means you would not write something like this: arrayName.sort(defaultSort()); The function call after the function name will cause an error because you don’t want to call the function. You only want to give the sort() method the function name so that it knows which function to use.<br /> <br /> Let’s look at an example of how to use this function we have created. See Listing 14.14. LISTING 14.14<br /> <br /> INPUT<br /> <br /> Using the defaultSort() Function with the Array Object’s sort() Method (SortStringsAndNumbers03.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the sort() method on numbers and strings</title> <script type=”text/javascript” language=”javascript”> <!-- //<br /> <br /> Advanced Array Management<br /> <br /> LISTING 14.14<br /> <br /> 479<br /> <br /> continued<br /> <br /> function defaultSort(elementX, elementY) { if (elementX < elementY) return -1; if (elementX > elementY) return 1; return 0; } function SortElements() { var mixture = new Array(“red”,4,”blue”,2,”green”,9); mixture.sort(defaultSort); var mj = mixture.join(“, “); alert(“The sorted array contains: “ + mj); } // --> </script> </head> <body onload=”SortElements()”> </body> </html><br /> <br /> This brings up the alert box shown in Figure 14.14.<br /> <br /> OUTPUT FIGURE 14.14 A strange “sort.” Not quite what we were looking for.…<br /> <br /> It is at this point that we have to admit that the function above doesn’t quite produce the same results as the internal default function used by the sort() method. The reason for this is that our function compares elements with the data type of the data in the array. The default comparison function on the other hand converts the data type of the elements it is sent with their data type set as a string (so that it can do a lexicographical sort).<br /> <br /> ANALYSIS<br /> <br /> There is a reason why having a mixture of data types causes problems for our function— it is a feature of the comparison operators that we haven’t looked at before. When the operands of the comparison functions have different data types the value returned is (almost) always the value false. Therefore: 1 < “a” “a” < 1<br /> <br /> // evaluates to false // evaluates to false<br /> <br /> 14<br /> <br /> 480<br /> <br /> Day 14<br /> <br /> The result of this is that our comparison function can send the sort() method contradictory commands. Inevitably this results in ordering that at times can seem almost random. To prevent this problem from occurring, we need the elements that are sent to our comparison functions to be compared with the same data type. To do this for our previous function we might write it like this: function defaultSort(elementX, elementY) { var element1 = elementX.toString(); var element2 = elementY.toString(); if (element1 < element2) return -1; if (element1 > element2) return 1; return 0; }<br /> <br /> In this modified function, we have created two temporary variables called element1 and element2 (notice the slight change in name). To these variables we have assigned the data from the two arguments after converting their data to the datatype string using the toString() method. It is then these copies with the modified data type that we use for the comparison to decide which value to return to the sort() method. This allows us to have a mixture of data with the datatypes number, string and boolean in our arrays, and they will be ordered lexicographically. Listing 14.15 shows a revised example.<br /> <br /> INPUT<br /> <br /> LISTING 14.15<br /> <br /> A Custom Function Produces the Same as the Default<br /> <br /> sort() Method (SortStringsAndNumbers04.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using the sort() method on numbers and strings</title> <script type=”text/javascript” language=”javascript”> <!-- // function defaultSort(elementX, elementY) { var element1 = elementX.toString(); var element2 = elementY.toString(); if (element1 < element2) return -1; if (element1 > element2) return 1; return 0; } function SortElements() { var mixture = new Array(“red”,4,”blue”,2,”green”,9); mixture.sort(defaultSort);<br /> <br /> Advanced Array Management<br /> <br /> LISTING 14.15<br /> <br /> 481<br /> <br /> continued<br /> <br /> var mj = mixture.join(“, “); alert(“The sorted array contains: “ + mj); } // --> </script> </head> <body onload=”SortElements()”> </body> </html><br /> <br /> When you run Listing 14.15 it will produce the alert box shown in Figure 14.15.<br /> <br /> OUTPUT FIGURE 14.15 The output of a modified defaultSort()<br /> <br /> function.<br /> <br /> Although the functions we have seen so far have introduced us to the use of the comparison operators in comparison functions, and the need for conversion to a consistent data type, they are not really of much use. After all we don’t need to create a function that will produce the same results as the sort() method produces by default. Far more useful would be functions that will produce a case-insensitive sort and a numerical sort. Let’s go on now to create both of these.<br /> <br /> ANALYSIS<br /> <br /> Hopefully by now you have grasped the idea of the arguments of our comparison functions containing the data from two array elements. We will now drop the longer names we have been using and use the traditional shorter argument names for comparison function arguments: a and b. The first function we are going to look at is one that causes a case-insensitive alphabetical sort. In the previous example, we converted the arguments of the function so that they had the datatype string. In this function we are going to make a slightly more significant modification to the data and convert it to lowercase letters. By comparing the arguments with all their letters reduced to lowercase, the case of the characters in the actual array elements doesn’t play a part:<br /> <br /> 14<br /> <br /> 482<br /> <br /> Day 14<br /> <br /> function alphabetical(a, b) { var A = a.toLowerCase(); var B = b.toLowerCase(); if (A < B) return -1; if (A > B) return 1; return 0; }<br /> <br /> Notice that the temporary variables are uppercase, whereas the argument names are lowercase. Because JavaScript is case sensitive the arguments are not changed. We could just as well have used the string method toUpperCase() rather than the method toLowerCase(); both would stop the uppercase letters from being separated from lowercase letters. For example, if we wanted to make a case-insensitive sort of the colors we were using before, we would use code like that in Listing 14.16 (see Figure 14.16).<br /> <br /> INPUT<br /> <br /> LISTING 14.16 An Alphabetical String Sort, Independent of Case (SortStringsAndNumbers05.htm)<br /> <br /> <!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “DTD/xhtml1-transitional.dtd”> <html> <head> <title>Using an alphabetical sort() method on strings</title> <script type=”text/javascript” language=”javascript”> <!-- // function alphabetical(a, b) { var A = a.toLowerCase(); var B = b.toLowerCase(); if (A < B) return -1; if (A > B) return 1; return 0; } function SortElements() { var colors = new Array(“red”,”Red”,”blue”,”Blue”,”green”,”Green”); colors.sort(alphabetical); var cj = colors.join(“, “); alert(“The sorted array contains: “ + cj); } // --> </script> </head><br /> <br /> Advanced Array Management<br /> <br /> LISTING 14.16<br /> <br /> 483<br /> <br /> continued<br /> <br /> <body onload=”SortElements()”> </body> </html><br /> <br /> OUTPUT FIGURE 14.16 A case-independent, alphabetical string sort.<br /> <br /> As you can see from the alert box, this time color names have not been separated by the case of their initial letter—upper and lowercase characters have been treated equally.<br /> <br /> ANALYSIS<br /> <br /> Note<br /> <br /> The ordering of words equal in a case-insensitive comparison will depend on their relative position in the initial ordering of the array.<br /> <br /> Let’s now look at how we can create a sort for numerical values. It is as simple as the following code: function numerical(a, b) { return a - b; }<br /> <br /> There are two points to note about these two comparison functions. The case-insensitive comparison function will result in an error, if the array it is used on has any data that has a datatype other than a string. This is because the toLowerCase() method is a method belonging to the String object and therefore can’t be used with other datatypes. This could be partially remedied by applying the toString() method prior to the toLowerCase() method. This is shown in the code below: function alphabetical(a, b) { var A = a.toString().toLowerCase(); var B = b.toString().toLowerCase(); if (A < B) return -1; if (A > B) return 1; return 0; }<br /> <br /> 14<br /> <br /> 484<br /> <br /> Day 14<br /> <br /> This will allow numerical and boolean values to be present in the array on which the function is used. Most of the time when you are doing a case-insensitive alphabetical sort, these values will not be present. But for completeness, you may want to use that form of the alphabetical() function. The numerical comparison function may also result in an error (or at least unexpected results); but this time it would be because the array it is used on contains data that is not numerical. For example, 10 – “a”<br /> <br /> evaluates to NaN. Again this isn’t an issue if the array you will use the numerical() function with only contains numerical data.<br /> <br /> Multidimensional Arrays The arrays we have been working with so far have been single dimension arrays. By this we mean that you can think of them as a single column of data. There is another type of array that we may wish to work with that can be thought of as not only containing columns of data but also rows. These types of arrays are called two-dimensional arrays. In most full-programming languages, the syntax for these types of arrays is written as in the following example: myElement[x,y]<br /> <br /> // Not how it’s done in JavaScript!!<br /> <br /> This refers to an element y that can be thought of as belonging to x (where x and y are index numbers). JavaScript doesn’t directly support two-dimensional arrays but, with a clever little trick, we can simulate two-dimensional arrays in JavaScript. To do this we need to store arrays within the elements of another array. Effectively, we then have an array of arrays. This is probably easier to understand with an example, so let’s take a look at one: var TwoDArray = new Array(); TwoDArray[0] = new Array(); TwoDArray[1] = new Array();<br /> <br /> In the piece of code above we have created an array called TwoDArray in the normal way with the new operator and the Array() constructor function. However, instead of assigning a piece of data to its first and second elements, we have assigned them two new arrays. These arrays are effectively stored within the elements of TwoDArray. To see how this is similar to a two-dimensional array, let’s give the two arrays stored within the elements of the array TwoDArray some data:<br /> <br /> Advanced Array Management<br /> <br /> 485<br /> <br /> var TwoDArray = new Array(); TwoDArray[0] = new Array(); TwoDArray[1] = new Array(); TwoDArray[0][0] TwoDArray[0][1] TwoDArray[1][0] TwoDArray[1][1]<br /> <br /> = = = =<br /> <br /> “a”; “b”; “c”; “d”;<br /> <br /> At first this may look a little complex, but if you take time to think it through hopefully you will see what’s happening in the four new lines. In the first of these lines the part TwoDArray[0] is simply a reference to the array we stored in the first element of TwoDArray. Therefore, the extra [0] is a reference to the first element of the array stored in the first element of TwoDArray. It is this element of the array—stored in the first element of TwoDArray—that is assigned the value a. The same happens for the second element of the array stored in the first element of the array TwoDArray, except that it is given the value b. In the third line, we move to the second element of TwoDArray. Remember this element contains a different array from the one in the first element. First we assign its first element the value c, then in the next line we assign its second element the value d. If it helps you to understand two-dimensional arrays, you can think of them as a table. The number in the first set of square brackets would represent the row number, and the number in the second set of square brackets would represent the column number. In the example we have just looked at, this would mean that row 1 would have the values a and b in columns 1 and 2 respectively, and row 2 would have the values c and d in columns 1 and 2 respectively. The benefit of ordering our data in two-dimensional arrays is that it gives us an extra level of detail. The idea of containing arrays in the elements of another array can be extended indefinitely. We are not limited simply to two dimensions. We can have as many levels of arrays stored in elements of arrays as needed for a given task. In general, when we store arrays in the elements of another array we call them multidimensional arrays. JavaScript multidimensional arrays are really just arrays where the indexes are references to other arrays. Before we finish, let’s look at a more involved example where multidimensional arrays are used in conjunction with the Array object’s sort() method to order a list of e-mails. This example assumes that a server-side process has filled the arrays with the latest four e-mails. Take a look at Listing 14.17. The initial output is shown in Figure 14.17.<br /> <br /> 14<br /> <br /> 486<br /> <br /> Day 14<br /> <br /> INPUT<br /> <br /> LISTING 14.17<br /> <br /> Using Multidimensional Arrays and Sorting on Screen<br /> <br /> (emailArray.htm)<br /> <br /> <html> <head> <title>My Emails</title> <style type=”text/css”> th {background-color: #999999} tr.odd {background-color: #cccccc} tr.even {background-color: #ffffff} th, td {padding: 0.1em 1em} </style> <script language=”javascript” type=”text/javascript”> <!-var emails<br /> <br /> = new Array();<br /> <br /> for (var i=0; i<4; i++) { emails[i] = new Array(); } emails[0][“From”] = “Iain”; emails[0][“Date”] = “2001-07-31”; emails[0][“Subject”] = “TV Tonight”; emails[1][“From”] = “Member’s Services”; emails[1][“Date”] = “2001-07-30”; emails[1][“Subject”] = “Welcome”; emails[2][“From”] = “Lottery”; emails[2][“Date”] = “2001-07-29”; emails[2][“Subject”] = “You’ve Won!!”; emails[3][“From”] = “Unwanted Spam”; emails[3][“Date”] = “2001-07-28”; emails[3][“Subject”] = “Annoying Email”; function sortByFrom(a, b) { var A = a[“From”].toLowerCase(); var B = b[“From”].toLowerCase(); if (A < B) return -1; if (A > B) return 1; return 0; } function sortByDate(a, b) {<br /> <br /> Advanced Array Management<br /> <br /> LISTING 14.17<br /> <br /> 487<br /> <br /> continued<br /> <br /> var A = a[“Date”]; var B = b[“Date”]; if (A < B) return -1; if (A > B) return 1; return 0; } function sortBySubject(a, b) { var A = a[“Subject”].toLowerCase(); var B = b[“Subject”].toLowerCase(); if (A < B) return -1; if (A > B) return 1; return 0; } var sortCriteria = location.search.substr(1); if (sortCriteria) { emails.sort(eval(sortCriteria)); } //--> </script> </head> <body> <h1>My Emails</h1> <table> <tr> <th><a href=”?sortByFrom” rel="nofollow">From</a></th> <th><a href=”?sortByDate” rel="nofollow">Date</a></th> <th><a href=”?sortBySubject” rel="nofollow">Subject</a></th> </tr> <script language=”javascript” type=”text/javascript”> <!-for (var i=0; i<emails.length; i++) { document.write(“ <tr class=’” + ((i%2)? “odd”: “even”) + “‘>”); document.write(“ document.write(“ document.write(“ document.write(“ }<br /> <br /> <td>” + emails[i][“From”] + “</td>”); <td>” + emails[i][“Date”] + “</td>”); <td>” + emails[i][“Subject”] + “</td>”); </tr>”);<br /> <br /> 14<br /> <br /> 488<br /> <br /> Day 14<br /> <br /> LISTING 14.17<br /> <br /> continued<br /> <br /> //--> </script> </table> </body> </html><br /> <br /> OUTPUT FIGURE 14.17 Sorting on screen using multidimensional arrays.<br /> <br /> On the very first line of JavaScript we created an array called emails. Each element of the emails array will contain information about an e-mail. However, because we want to have several pieces of information about each e-mail, such as who it’s from and when it arrived, we can’t just put pieces of data straight into this array. Instead we insert new arrays into each of the elements of the emails array (in this case we have used a for loop to do all four assignments). This allows us to associate several pieces of information with each e-mail entry.<br /> <br /> ANALYSIS<br /> <br /> The next four blocks of three lines each assign a piece of data to associate elements (remember, elements with a name instead of a number). We could have used numerical indexes but using names makes it easier to read for this example. A piece of data is assigned to elements called “From”, “Date”, and “Subject” for each of our four e-mails. For the moment ignore the rest of the code in the script block in the head and skip to the body. Here we create a table with three headings: one for each e-mail field. Note that these headings are wrapped with links with href attributes containing a short query string. When clicked on, these links will simply reload the document with the query string added to the end of the URL. We will see why shortly.<br /> <br /> Advanced Array Management<br /> <br /> 489<br /> <br /> To write out the table’s contents, we use a script block containing a for loop. This loop is set to loop through the array called emails and write the data for each e-mail into the table in its own row. The first line in the loop is probably the one you are least likely to understand, so let’s take a closer look at it now: document.write(“<br /> <br /> <tr class=’” + ((i%2)? “odd”: “even”) + “‘>”);<br /> <br /> The line writes a <tr> in the following form: <tr class=’xxx’><br /> <br /> The xxx part is determined by the modulo expression within the conditional operator. If i is even, then the expression evaluates to 0; but if i is odd, then it evaluates to 1. Because JavaScript understands 0 to be false and 1 to be true in logical contexts, the conditional operator alternates between evaluating as “odd” and “even” with each loop. By alternating the class of each row, we can assign a different background color to help each e-mail stand out. The first time the page loads the e-mails are written into the table in the order in which they are contained within the array called emails. But if we click on one of the headers we can change this. To understand how this works, let’s look at how the URL might change if we clicked on the From header: http://www.xmml.com/email/index.htm?sortByFrom<br /> <br /> Let’s now look at the code contained within the document head that we missed earlier, at the last line to be more precise. As we said in an earlier chapter, by using the location.search property of the window object, we can obtain the query string from the URL. However, this returns the query string with the question mark still in place. To remove this, we use the substr() method. In effect the following line retrieves the query string and removes the question mark so that we can assign the rest of the query string to the variable sortCriteria, like so: var sortCriteria = location.search.substr(1);<br /> <br /> Notice that the query string values given to the three-header links correspond to the names of the three functions that precede the above line. When the value after the question mark in the query string has been assigned to the variable sortCriteria, an if statement checks to see whether the variable has any value. If it does the following line is executed: emails.sort(eval(sortCriteria));<br /> <br /> The eval() function is the first thing to evaluate in this statement. It takes the function name that is stored in the sortCriteria variable and interprets it as code. Therefore the function name is passed as a parameter to the sort() method, as it is applied to the array<br /> <br /> 14<br /> <br /> 490<br /> <br /> Day 14<br /> <br /> called emails. In the case of our example, where the From header is clicked, it is the name of the following function: function sortByFrom(a, b) { var A = a[“From”].toLowerCase(); var B = b[“From”].toLowerCase(); if (A < B) return -1; if (A > B) return 1; return 0; }<br /> <br /> This function tells the sort() method that rather than basing the sort on the contents of the elements of the array it is applied to, it should look at an element of the array contained within that element. In this case, the element it should base the sort on is the “From” element. The values of this field are also converted to lowercase for the check to make the sort case insensitive. The conditions are then as we saw earlier in the chapter. If the first element comes before the other in the alphabet, then –1 is returned to indicate it should not be swapped. But if the first element comes after the other in the alphabet, then 1 is returned to indicate that it should be swapped. If neither of these conditions are met, then the elements must be identical and so 0 is returned to indicate they do not need swapped. Note that in this case that when a swap does take place, then all that is contained within the two elements being compared is swapped. Therefore in the case of these elements containing arrays, the whole arrays are moved—not just parts of them. This will keep all of the information connected with one e-mail together. Once the sort of the emails array has taken place the rest of the page finishes loading. This time when the table is generated from the array, it has already been sorted into the order that was selected by clicking on the header. Of course if we are going to reload the page to restructure it, we might as well omit the JavaScript and get the server to order the e-mails and write them into a table itself. This isn’t the point though. You will learn techniques elsewhere in the book that will enable you to restructure the page without reloading and hence have virtually instant ordering. The point of this example was to show how a multidimensional array can be useful, and as an added benefit, how the sort method can be used with multidimensional arrays.<br /> <br /> Advanced Array Management<br /> <br /> 491<br /> <br /> Summary Always look out for ways of using arrays to structure your data so that your scripts can run more efficiently. Whenever you have a script that performs a repeated operation on some data, ask yourself if this data wouldn’t be better suited as an array so a loop statement could make this process easier. For example when you have elements of a form that must have the same check made, then give them all the same name so that you can treat them as an array. This will save you writing out individual lines of code for processing each one (hard-coding).<br /> <br /> Workshop In this workshop we will review what you have learned about advanced arrays in this chapter.<br /> <br /> Q&A Q. Are all the Array object’s methods cross-browser? A. No. Some methods, such as the splice() method are new to one browser. The splice() method was introduced in JScript 5.5, and with Internet Explorer 5.5, so you may want to avoid its use for a little time until most of the users of your site have upgraded to Internet Explorer 5.5 or higher. The splice() method has been available to Netscape since version 4. Q. If you sort “100” and “85” as strings will “85” be less than “100”? A. No. When sorted as strings the character “1” comes before the character “8”. Therefore “100” is “less” than “85”. Of course, if these two values were sorted as numbers, you would have the opposite result. Q. Can you make a copy of an array by assigning the array to a new variable? A. No. If you were to attempt this, the new variable would only contain a copy of a reference to the area in memory in which the array is stored. Any changes made to the array are also reflected if accessed using the new variable.<br /> <br /> Quiz 1. Which method is used to concatenate two arrays? 2. Which method of the Array object allows array elements to be deleted, inserted, and/or replaced? 3. Which method is used to make a true copy of an array?<br /> <br /> 14<br /> <br /> 492<br /> <br /> Day 14<br /> <br /> Quiz Answers 1. The concat() method is used to concatenate two arrays. 2. The splice() method. The second argument of the splice() method controls how many elements are deleted. If further arguments are present, then new elements are added to the array. Whether the length property increases, decreases, or stays the same depends on how many deletions and insertions are made. 3. The concat() method can be used to make a true copy of an array by concatenating that array with an empty array.<br /> <br /> Exercises 1. Create an array that contains numerical values in each element of the array, and then sort the array by numerical value. 2. Create a two-dimensional array for two locations, with maximum daily temperature for each of seven days of the week. Sort the data by temperature value.<br /> <br /> WEEK 3 Advanced Topics 15 Debugging and Error Handling 16 Cookies: Storing Persistent Data 17 Privacy and Security 18 Plugins and Applets 19 Creating Your Own Objects 20 JavaScript in E-Commerce 21 JavaScript and SVG<br /> <br /> WEEK 3<br /> <br /> DAY<br /> <br /> 15<br /> <br /> Debugging and Error Handling The people who put together Web pages run the gamut from professional programmers, who are accustomed to working with computers and computer languages, to those whom a Web page is their first exposure to “programming.” The HTML used to put together Web pages is, as the “ML” in HTML signifies, a “markup language” that is relatively straightforward to use. When you leave the safe confines of HTML, however, and embark into JavaScript programming, you are using a real programming language. As an interpreted language, JavaScript is a lot easier to get into than languages such as C++ or Java, but as soon as you go beyond cutting and pasting “prefab” JavaScript directly into your pages, you are programming. Like with all programming, you will quickly discover the “joy” of dealing with programming errors and bugs. There are not very many development tools specifically designed for debugging JavaScript, but there are a few. Additionally, there are a number of techniques that can be used to debug your own JavaScript programs.<br /> <br /> 496<br /> <br /> Day 15<br /> <br /> This chapter will show you how to do the following: • Use some good programming practices to cut down on errors • Use document.writeln() to output debugging information • Use alert boxes to step through JavaScript programs • Add HTML form fields to display debugging information • Use some dedicated script debugging tools<br /> <br /> Preventing and Classifying JavaScript Errors Ideally, the best way to “debug” JavaScript programs is to write error-free code. Unfortunately, this is nearly impossible to do on a consistent basis. It is possible, however, to develop coding practices and to become familiar with the different types of errors, and in doing so help make your JavaScript code run as desired. The field of software engineering is concerned with many aspects of computer programming—including the development of coding practices that minimize programming errors. Obviously, in these few pages, these topics cannot be covered fully. However, some topics can be discussed and techniques can be shown that will give you the necessary tools for creating and debugging JavaScript code.<br /> <br /> Commenting Code If you’ve ever read any book on programming, one of the first recommendations is that you comment your code. Obviously, simply adding comments to your programs is not going to prevent errors, but it does mean that you will need to put some thought into how your programs are constructed, which may help you catch logic or other errors. The real benefit of commenting code, though, happens when you come back to the code at a later time. It is a common practice in programming to start with an older program and then change it when doing something new, rather than starting from scratch. It is in a situation like this when you will be glad that you have commented your code. Wellcommented and documented code makes it easier for you to remember what a given JavaScript does, and also will make it much easier for you to adapt your code for other uses.<br /> <br /> Note<br /> <br /> The topic of commenting your code is usually one of those, “Do what I say, not what I do” sort of things. A lot of the examples in this book may not be commented as well as they should; still, it is a good practice.<br /> <br /> Debugging and Error Handling<br /> <br /> 497<br /> <br /> Keeping Track of JavaScript Variables A common source of errors in JavaScript programs comes from its fairly lax requirements concerning variables. JavaScript does not require variables to be strongly typed, so the same variable can be used to hold integers, floating-point numbers, Boolean values, and strings. JavaScript does a fairly good job of automatic conversion from one data type to another when performing various operations, but there is still the possibility of problems during conversion (particularly with the addition operator +, which is also used for string concatenation). There are a number of techniques and approaches that can be used to minimize the potential pitfalls of JavaScript variable problems. These include the following: • Explicit casting—Rather than relying on JavaScript’s automatic data-type conversion, use explicit casting to convert values before operating on them. To convert to an integer or floating-point number, use the int() or float() functions, respectively. To convert a number to a string, you can use the toString() method of the Math object. The eval() function also can be used to turn numeric strings into numbers. • Variable declaration—The JavaScript var statement is used to declare variables, but it is not required. Variables can be created on-the-fly by simply assigning a value to them. By explicitly using the var statement to declare each variable, you will be able to keep better track of them. • Local versus global variables—Similar to the explicit use of the var statement, it is important to remember the scope of variables created. When declared with the var statement inside a function, a variable is local to that function and cannot be accessed elsewhere. When a variable is declared outside of a function, it is a global variable and can be accessed in any script. Conflicts between global and local variables are a common source of errors, particularly when outside scripts are loaded using the src attribute of the <script> tag. • Variable-naming convention—By creating a variable naming convention, you can make it much easier to keep track of all of your variables, particularly when there are a lot of them. One way you can keep track of your variables is to use a given prefix to identify integers, floating-point numbers, Booleans, and strings. If you are including and using external JavaScripts, you also might consider prefixing all of your variables with a given prefix (such as your initials), in order to reduce the possibility of conflicts with any variables in the external script.<br /> <br /> 15<br /> <br /> 498<br /> <br /> Day 15<br /> <br /> JavaScript Error Types There are several different types of errors that are commonly encountered when creating any program, including JavaScript. How each type of error is found and dealt with is slightly different. The most common types of errors are the following: • Syntax errors—In the initial creation of any program, the first type of error that you will encounter is a syntax error. This most commonly happens because of a typo you may have inserted into the program when typing a JavaScript statement, either in the spelling of the keyword itself or by mis-nesting braces or parentheses. As will be shown below, some browsers are able to help you pinpoint where these errors are, and once found they usually are not too hard to correct. Typically, syntax errors prevent a script from running. • Logic errors—This catch-all classification covers the bulk of errors, and they are the hardest types of errors to discover. There are even cases where “syntax” type errors cause logic errors, since a typo in a variable name will not necessarily keep the program from running. Correcting the logic in a program to get it to run correctly is at the heart of computer programming. • Browser-based errors—Each Web browser has slightly different capabilities, and supports different versions of JavaScript. It is quite possible that a JavaScript that works fine in one browser will not work well in another, either because of differences in the implementation of JavaScript or because of the differences in the capabilities of the underlying browser. The three most prominent browsers, Internet Explorer, Netscape Navigator 6.x, and Netscape Navigator 4.x, are largely compatible, but they also have significant differences from each other. Additionally, older versions of all of these browsers (as well as other JavaScript-enabled browsers such as Opera) also have some differences. There are strategies for programming multiple browsers to avoid these types of errors. These strategies were covered in Chapter 8, “The Browser Issue,” and Chapter 11, “Dynamic HTML.”<br /> <br /> Finding Syntax Errors In most computer languages, a syntax error will prevent a program from running. Often you will be informed of where the problem is, which generally makes finding and correcting syntax errors fairly simple. After writing a program, you continue trying to run it until it actually runs. At that point, the syntax errors (at least the syntax errors in the portion of the program that you have run) will have all been corrected. In many cases, a JavaScript interpreter treats syntax errors the same way. The JavaScript interpreter that actually runs a JavaScript program is part of the Web browser used to<br /> <br /> Debugging and Error Handling<br /> <br /> load the HTML document that includes and calls it. The JavaScript interpreters in Netscape Navigator and Internet Explorer have different reactions to syntax errors, however.<br /> <br /> Errors in Netscape Navigator Figure 15.1 shows the reaction of Netscape Navigator 6 to an HTML document that contains JavaScript, which includes a syntax error that prevents correct execution. The example program is one shown in greater detail in the next section; if the program were working, though, there would be output between the two horizontal rules. As can be seen from the figure, however, there is no outward indication that there is a syntax error, other than the lack of output. FIGURE 15.1 Syntax errors in Netscape Navigator are not always reported.<br /> <br /> Netscape Navigator has a JavaScript console that can be displayed in order to get information about JavaScript errors. In order to display the console, type javascript: into the address line of the browser, and a window similar to that shown in Figure 15.2 will display. FIGURE 15.2 Netscape’s JavaScript console gives error information.<br /> <br /> As shown, the JavaScript console gives information about the different types of errors that occurred. In this case, it is possible to pinpoint the syntax error (a missing close quotation mark on line 54) by looking at these error messages. It is interesting to note<br /> <br /> 499<br /> <br /> 15<br /> <br /> 500<br /> <br /> Day 15<br /> <br /> that a single syntax error caused both of the error messages shown in Figure 15.2. This is because the first error, the one listed as being in line 54, caused the definition of the timeSpelledOut function to be incomplete and undefined, causing the error from line 63.<br /> <br /> Errors in Internet Explorer Similar to Netscape Navigator, Internet Explorer also allows you to examine error messages caused by syntax errors in JavaScript programs. In fact, Internet Explorer makes it a little more obvious that an error has occurred, as shown in Figure 15.3. In addition to there not being any script output (as expected), Internet Explorer uses a small icon on the left side of the status bar to indicate an error. (Sometimes, a line such as “Done, but errors have occurred” will also appear in the status line.) FIGURE 15.3 By default, Internet Explorer uses a small status bar icon to indicate a script error.<br /> <br /> When the error icon appears, you can display an Error dialog box by double-clicking on the error icon. The Error dialog box will appear, as shown in Figure 15.4. The details of the error can be examined, as with Netscape’s JavaScript console, to determine where the JavaScript error occurred. Note that the Previous and Next buttons (when enabled) on the bottom of the Error dialog box allow you to scroll through all of the error messages generated. As shown above, it is possible for a single error to generate more than one error message; in that case, it is important to scroll through all of the messages in order to find the exact source of the error.<br /> <br /> Debugging and Error Handling<br /> <br /> 501<br /> <br /> FIGURE 15.4<br /> <br /> 15<br /> <br /> Double-clicking the error icon will display the Error dialog box.<br /> <br /> Note<br /> <br /> Notice the check box in the Error dialog box shown in Figure 15.4, “Always display this message when a page contains errors.” If you check this box, any JavaScript (or other Web page) errors in the future will automatically bring up the Error dialog box. This optional setting can also be made using the Advanced tab of the Internet Options, checking the box for “Display a notification about every script error.”<br /> <br /> Debugging Load-Time Errors For the most part, in the rest of the chapter you will see different techniques that you can use to debug JavaScript errors. Different types of JavaScript require different approaches to debugging; in the next three sections, three different types of scripts will be discussed. The first type of JavaScript that will be discussed is a load-time script. This is a JavaScript that is loaded and executed when the Web page first loads in the browser. Listing 15.1 shows an example of such a Web page and JavaScript program.<br /> <br /> INPUT<br /> <br /> LISTING 15.1<br /> <br /> Load-Time Script for “Formatted” Time (timeExample.html)<br /> <br /> <html> <head> <title>timeSpelledOut</title> <script language=”javascript” type=”text/javascript”> function timeSpelledOut() { var now = new Date(); var hour = now.getHours(); var minute = now.getMinutes(); var hourn,minuten,minute_frac,little_str,big_str; /*<br /> <br /> 502<br /> <br /> Day 15<br /> <br /> LISTING 15.1<br /> <br /> continued<br /> <br /> * figure out what two numbers the little hand are between */ if (hour >= 12) hour -= 12; if (hour == 0) { hour = 12; hourn = 1; } else hourn = hour + 1; /* * figure out what two numbers the big hand are between */ minute /= 5; minute_frac = minute - Math.floor(minute); minute = Math.floor(minute); if (minute == 0) { minute = 12; minuten = 1; } else minuten = minute + 1; /* * spell out where the little and big hands are */ if (minute == 12 && minute_frac < 0.1) little_str = “The little hand is on the “ + hour; else if (minute == 11) little_str = “The little hand is almost on the “ + hourn; else if (minute == 12 || minute == 1) little_str = “The little hand is just past the “ + hour; else little_str = “The little hand is between the “ + hour + “ and the “ + hourn; if (minute_frac == 0.0) big_str = “The big hand is on the “ + minute; else if (minute_frac > 0.7) big_str = “The big hand is almost on the “ + minuten; else if (minute_frac < 0.3) big_str = “The big hand is just past the “ + minute; else big_str = “The big hand is between the “ + minute + “ and the “ + minuten; /* * return time spelled out */ return little_str + “...<br>” + big_str + “...”; } </script> </head> <body><br /> <br /> Debugging and Error Handling<br /> <br /> LISTING 15.1<br /> <br /> 503<br /> <br /> continued<br /> <br /> <center> <h1>timeSpelledOut</h1> <hr> <script language=”javascript”> document.writeln(“<h2>” + timeSpelledOut() + “</h2>”); </script> <hr> </center> <em>Jim O’Donnell, <a href=”mailto:jim@odonnell.org” rel="nofollow">jim@odonnell.org</a></em> </body> </html><br /> <br /> The JavaScript included in Listing 15.1 displays a formatted time when the Web page is loaded. This particular format, known as “time spelled out,” gives the current time as it would be displayed on an analog clock. Figure 15.5 shows an example of the current time, “spelled out.”<br /> <br /> OUTPUT FIGURE 15.5 Load-time JavaScript that outputs “analog” time.<br /> <br /> Using the document.writeln() Method Because a load-time JavaScript executes while the Web page is writing out, the easiest way to get debugging information (to get some insight into the workings of the script), is to use the document.writeln() method to write out additional information directly into the page. Listing 15.2 shows the same HTML document and JavaScript as that shown in Listing 15.1, only this time with additional debugging statements included to write out intermediate information from the script. JavaScript comments are used to highlight the<br /> <br /> 15<br /> <br /> 504<br /> <br /> Day 15<br /> <br /> debugging information included, so that the statements may be removed (or commented out easily when the script is working correctly). Figure 15.6 shows an example of the resulting Web page that shows both the intended output as well as the debug output that shows intermediate values.<br /> <br /> INPUT<br /> <br /> LISTING 15.2<br /> <br /> Using document.writeln() to Write Debugging Information<br /> <br /> <html> <head> <title>timeSpelledOut</title> <script language=”javascript” type=”text/javascript”> function timeSpelledOut() { var now = new Date(); var hour = now.getHours(); var minute = now.getMinutes(); var hourn,minuten,minute_frac,little_str,big_str; /* * figure out what two numbers the little hand are between */ if (hour >= 12) hour -= 12; if (hour == 0) { hour = 12; hourn = 1; } else hourn = hour + 1; /* * figure out what two numbers the big hand are between */ minute /= 5; minute_frac = minute - Math.floor(minute); minute = Math.floor(minute); if (minute == 0) { minute = 12; minuten = 1; } else minuten = minute + 1; /* * spell out where the little and big hands are */ if (minute == 12 && minute_frac < 0.1) little_str = “The little hand is on the “ + hour; else if (minute == 11) little_str = “The little hand is almost on the “ + hourn; else if (minute == 12 || minute == 1) little_str = “The little hand is just past the “ + hour;<br /> <br /> Debugging and Error Handling<br /> <br /> LISTING 15.2<br /> <br /> 505<br /> <br /> continued<br /> <br /> else little_str = “The little hand is between the “ + hour + “ and the “ + hourn; if (minute_frac == 0.0) big_str = “The big hand is on the “ + minute; else if (minute_frac > 0.7) big_str = “The big hand is almost on the “ + minuten; else if (minute_frac < 0.3) big_str = “The big hand is just past the “ + minute; else big_str = “The big hand is between the “ + minute + “ and the “ + minuten; /* * DEBUG: output intermediate variables */ document.writeln(“now = “ + now + “<br>”); document.writeln(“hour = “ + hour + “<br>”); document.writeln(“hourn = “ + hourn + “<br>”); document.writeln(“minute = “ + minute + “<br>”); document.writeln(“minuten = “ + minuten + “<br>”); document.writeln(“minute_frac = “ + minute_frac + “<br>”); document.writeln(“little_str = “ + little_str + “<br>”); document.writeln(“big_str = “ + big_str + “<br>”); /* * return time spelled out */ return little_str + “...<br>” + big_str + “...”; } </script> </head> <body> <center> <h1>timeSpelledOut</h1> <hr> <script language=”javascript”> document.writeln(“<h2>” + timeSpelledOut() + “</h2>”); </script> <hr> </center> <em>Jim O’Donnell, <a href=”mailto:jim@odonnell.org” rel="nofollow">jim@odonnell.org</a></em> </body> </html><br /> <br /> 15<br /> <br /> 506<br /> <br /> Day 15<br /> <br /> OUTPUT FIGURE 15.6 Debugging information can be written directly into the Web page.<br /> <br /> Using the Alternate Inputs The timeSpelledOut() JavaScript function shown in Listings 15.1 and 15.2 indicates a need for further techniques for debugging. Because the script is designed to print out a formatted version of the current time, it would take 24 hours to test all of the necessary combinations of times. Obviously, you don’t want to have to wait all that time to test all of the pertinent combinations. Listing 15.3 shows another alternative approach, one that allows for full testing of the function without requiring a great deal of time to hit all of the possibilities. As shown in the function, it is accomplished by supplying alternative inputs to the timeSpelledOut() function. When the function is called using these inputs, they are used in place of the actual time, thereby allowing for times other than the current time to be used. When the function is called without these inputs, the current time is used.<br /> <br /> INPUT<br /> <br /> LISTING 15.3<br /> <br /> Supplying Alternate Inputs to Debug Different Cases (timeExampleDebug2.html)<br /> <br /> <html> <head> <title>timeSpelledOut</title> <script language=”javascript” type=”text/javascript”> function timeSpelledOut(h_in,m_in) {<br /> <br /> Debugging and Error Handling<br /> <br /> LISTING 15.3 var var var var<br /> <br /> 507<br /> <br /> continued<br /> <br /> now = new Date(); hour = now.getHours(); minute = now.getMinutes(); hourn,minuten,minute_frac,little_str,big_str;<br /> <br /> /* * DEBUG: use function inputs, if defined */ document.writeln(“h_in “ + h_in + “<br>”); h_in = Math.floor(h_in); if (h_in >= 0 && h_in <= 23) hour = h_in; document.writeln(“m_in “ + m_in + “<br>”); h_in = Math.floor(m_in); if (m_in >= 0 && m_in <= 59) minute = m_in; /* * figure out what two numbers the little hand are between */ if (hour >= 12) hour -= 12; if (hour == 0) { hour = 12; hourn = 1; } else hourn = hour + 1; /* * figure out what two numbers the big hand are between */ minute /= 5; minute_frac = minute - Math.floor(minute); minute = Math.floor(minute); if (minute == 0) { minute = 12; minuten = 1; } else minuten = minute + 1; /* * spell out where the little and big hands are */ if (minute == 12 && minute_frac < 0.1) little_str = “The little hand is on the “ + hour; else if (minute == 11) little_str = “The little hand is almost on the “ + hourn; else if (minute == 12 || minute == 1) little_str = “The little hand is just past the “ + hour; else little_str = “The little hand is between the “ + hour + “ and the “ + hourn;<br /> <br /> 15<br /> <br /> 508<br /> <br /> Day 15<br /> <br /> LISTING 15.3<br /> <br /> continued<br /> <br /> if (minute_frac == 0.0) big_str = “The big hand is on the “ + minute; else if (minute_frac > 0.7) big_str = “The big hand is almost on the “ + minuten; else if (minute_frac < 0.3) big_str = “The big hand is just past the “ + minute; else big_str = “The big hand is between the “ + minute + “ and the “ + minuten; /* * DEBUG: output intermediate variables */ document.writeln(“now = “ + now + “<br>”); document.writeln(“hour = “ + hour + “<br>”); document.writeln(“hourn = “ + hourn + “<br>”); document.writeln(“minute = “ + minute + “<br>”); document.writeln(“minuten = “ + minuten + “<br>”); document.writeln(“minute_frac = “ + minute_frac + “<br>”); document.writeln(“little_str = “ + little_str + “<br>”); document.writeln(“big_str = “ + big_str + “<br>”); /* * return time spelled out */ return little_str + “...<br>” + big_str + “...”; } </script> </head> <body> <center> <h1>timeSpelledOut</h1> <hr> <script language=”javascript”> document.writeln(“<h2>” + timeSpelledOut(11, 0) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11, 1) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11, 2) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11, 3) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11, 4) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11, 5) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11, 6) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11,54) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11,55) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11,56) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11,57) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11,58) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(11,59) + “</h2>”); document.writeln(“<h2>” + timeSpelledOut(12, 0) + “</h2>”); </script> <hr><br /> <br /> Debugging and Error Handling<br /> <br /> LISTING 15.3<br /> <br /> 509<br /> <br /> continued<br /> <br /> </center> <em>Jim O’Donnell, <a href=”mailto:jim@odonnell.org” rel="nofollow">jim@odonnell.org</a></em> </body> </html><br /> <br /> Figure 15.7 displays the output of a debugged version of the Web page, showing the output from many different combinations of input times, which are spelled out on the page.<br /> <br /> OUTPUT FIGURE 15.7 Use debugging techniques to quickly test many output possibilities. Only a small part of the output is shown.<br /> <br /> Debugging Run-Time Errors I: Discrete Events Many JavaScript programs do not run at load-time, however. Using the document.writeln() method is not as appropriate for debugging those types of scripts, since the page is not currently open for writing. A prime example of such a run-time script—one that runs after the page has completely loaded and been displayed—is a script for performing validation of HTML form inputs. An example of such a script is shown in Listing 15.4. This script takes a telephone number as input, ensures that it is a valid U.S. telephone number (seven or ten digits, with an optional leading 1), and then reformats and displays it back in the form. (An example of its operation is shown in Figures 15.8 and 15.9.)<br /> <br /> 15<br /> <br /> 510<br /> <br /> Day 15<br /> <br /> LISTING 15.4<br /> <br /> INPUT<br /> <br /> JavaScript to Format U.S. Telephone Number (phoneExample.html)<br /> <br /> <html> <head> <title>phoneNumberFormat</title> <script language=”javascript” type=”text/javascript”> function phoneNumberFormat(obj) { var num_str = obj.phoneNumber.value; var xChars = new Array(“ “,”(“,”)”,”+”,”-”,”.”, “I”,”N”,”V”,”A”,”L”,”D”,”!”); /* * remove all extraneous characters leaving only numbers */ num_str_save = num_str; for (i = 0;i < xChars.length;i++) { x = xChars[i]; while (num_str.indexOf(x) > -1) num_str = num_str.substring(0,num_str.indexOf(x)) + num_str.substring(num_str.indexOf(x) + 1, num_str.length) } /* * remove leading 1, if present */ if (num_str.indexOf(“1”) == 0) num_str = num_str.substring(1,num_str.length); /* * verify length of number, if valid */ if (num_str.length == 7) num_str = num_str.substring(0,3) + “-” + num_str.substring(3,7); else if (num_str.length == 10) num_str = “(“ + num_str.substring(0,3) + “) “ + num_str.substring(3,6) + “-” + num_str.substring(6,10); else if (num_str_save.indexOf(“ INVALID!”) > -1) num_str = num_str_save; else num_str = num_str_save + “ INVALID!”; /* * put formatted phone number back into the form */ obj.phoneNumber.value = num_str; } </script> </head> <body> <center> <h1>phoneNumberFormat</h1> <hr><br /> <br /> Debugging and Error Handling<br /> <br /> LISTING 15.4<br /> <br /> 511<br /> <br /> continued<br /> <br /> <form name=”formInfo”> <b>Input Phone Number</b> <input type=”text” size=”30” name=”phoneNumber” onChange=”phoneNumberFormat(document.formInfo)”> </form> <hr> </center> <em>Jim O’Donnell, <a href=”mailto:jim@odonnell.org” rel="nofollow">jim@odonnell.org</a></em> </body> </html><br /> <br /> OUTPUT FIGURE 15.8 JavaScript is often used to validate and format forms input.<br /> <br /> FIGURE 15.9 Using document.writeln()<br /> <br /> does not work as well because the page has completely loaded.<br /> <br /> Using the alert() Method The alert() method makes it very simple to output intermediate script values, which enables you to get insight into the workings of a script by looking at the intermediate values that were generated. If the answer is not what you expect, which is why you would need to perform the debugging in the first place, looking at intermediate values is the best way to figure out why things are not working. Using alert boxes also allows you to single step through a script.<br /> <br /> 15<br /> <br /> 512<br /> <br /> Day 15<br /> <br /> Listing 15.5 shows the telephone-number formatting example, with a series of alert boxes used to show the intermediate values. When the script function is called by the onChange() event of the form text box, it formats the telephone number entered into the field. Now, at each step in the process, an alert box pops up to show the current state of the formatted output.<br /> <br /> INPUT<br /> <br /> LISTING 15.5 Use Alert Boxes to Pop Up Important Debugging Information (phoneExampleDebug1.html)<br /> <br /> <html> <head> <title>phoneNumberFormat</title> <script language=”javascript” type=”text/javascript”> function phoneNumberFormat(obj) { var num_str = obj.phoneNumber.value; var xChars = new Array(“ “,”(“,”)”,”+”,”-”,”.”, “I”,”N”,”V”,”A”,”L”,”D”,”!”); /* * DEBUG: display num_str in alert box */ alert(“INPUT: num_str = “ + num_str); /* * remove all extraneous characters leaving only numbers */ num_str_save = num_str; for (i = 0;i < xChars.length;i++) { x = xChars[i]; while (num_str.indexOf(x) > -1) num_str = num_str.substring(0,num_str.indexOf(x)) + num_str.substring(num_str.indexOf(x) + 1, num_str.length) } /* * DEBUG: display num_str in alert box */ alert(“EXTRANEOUS CHARACTERS REMOVED: num_str = “ + num_str); /* * remove leading 1, if present */ if (num_str.indexOf(“1”) == 0) num_str = num_str.substring(1,num_str.length); /* * DEBUG: display num_str in alert box */ alert(“LEADING \”1\” REMOVED: num_str = “ + num_str); /* * verify length of number, if valid */<br /> <br /> Debugging and Error Handling<br /> <br /> LISTING 15.5<br /> <br /> 513<br /> <br /> continued<br /> <br /> if (num_str.length == 7) num_str = num_str.substring(0,3) + “-” + num_str.substring(3,7); else if (num_str.length == 10) num_str = “(“ + num_str.substring(0,3) + “) “ + num_str.substring(3,6) + “-” + num_str.substring(6,10); else if (num_str_save.indexOf(“ INVALID!”) > -1) num_str = num_str_save; else num_str = num_str_save + “ INVALID!”; /* * DEBUG: display num_str in alert box */ alert(“FORMATTED: num_str = “ + num_str); /* * put formatted phone number back into the form */ obj.phoneNumber.value = num_str; } </script> </head> <body> <center> <h1>phoneNumberFormat</h1> <hr> <form name=”formInfo”> <b>Input Phone Number</b> <input type=”text” size=”30” name=”phoneNumber” onchange=”phoneNumberFormat(document.formInfo)”> </form> <hr> </center> <em>Jim O’Donnell, <a href=”mailto:jim@odonnell.org” rel="nofollow">jim@odonnell.org</a></em> </body> </html><br /> <br /> Figures 15.10 through 15.13 show the debugging alert boxes that popped up in the example from Listing 15.5. Note that, in addition to showing the current value of the formatted telephone number, explanatory text is included to detail where in the process the script is.<br /> <br /> 15<br /> <br /> 514<br /> <br /> Day 15<br /> <br /> OUTPUT FIGURE 15.10 Alert boxes can be used to show intermediate values.<br /> <br /> FIGURE 15.11 Intermediate script values give insight into its operation.<br /> <br /> FIGURE 15.12 Use alert boxes to show how a script is progressing step by step.<br /> <br /> FIGURE 15.13 Alerts can be used to pinpoint script errors and show intermediate and final results.<br /> <br /> ANALYSIS While alert boxes work pretty well for stepping through scripts, they can be a<br /> <br /> problem if there are many steps to go through. Even though you will remove the alert boxes after the script is fully debugged, it can still be a pain to get through a full script if there are too many alert boxes. In such a case, there is a way you can include all of the debugging input without requiring your input to display an alert box at each step.<br /> <br /> Using HTML Form Elements Another way to output debugging information during the operation of a script is to use HTML forms—in particular, text and text-area boxes. The text contents of each of these elements can be accessed and changed via JavaScript. Just as separate alerts were used in Listing 15.5 to display intermediate calculations and values in the script, the same information can be displayed in a text-area box.<br /> <br /> Debugging and Error Handling<br /> <br /> Listing 15.6 shows another version of the telephone-number formatting example, this one using a text-area box to display debug information. Figure 15.14 shows the page before the script is run. Notice that the debug text-area box is included at the end of the Web page, after the “real” Web page content.<br /> <br /> INPUT<br /> <br /> LISTING 15.6<br /> <br /> TEXTAREA Boxes Can Display Lots of Information (phoneExampleDebug2.html)<br /> <br /> <html> <head> <title>phoneNumberFormat</title> <script language=”javascript” type=”text/javascript”> function phoneNumberFormat(obj) { var num_str = obj.phoneNumber.value; var xChars = new Array(“ “,”(“,”)”,”+”,”-”,”.”, “I”,”N”,”V”,”A”,”L”,”D”,”!”); /* * DEBUG: display num_str in alert box */ textareaDebugWrite(“INPUT: num_str = “,num_str); /* * remove all extraneous characters leaving only numbers */ num_str_save = num_str; for (i = 0;i < xChars.length;i++) { x = xChars[i]; while (num_str.indexOf(x) > -1) num_str = num_str.substring(0,num_str.indexOf(x)) + num_str.substring(num_str.indexOf(x) + 1,num_str.length) } /* * DEBUG: display num_str in alert box */ textareaDebugWrite(“EXTRANEOUS CHARACTERS REMOVED: num_str = “,num_str); /* * remove leading 1, if present */ if (num_str.indexOf(“1”) == 0) num_str = num_str.substring(1,num_str.length); /* * DEBUG: display num_str in alert box */ textareaDebugWrite(“LEADING \”1\” REMOVED: num_str = “,num_str); /* * verify length of number, if valid */ if (num_str.length == 7) num_str = num_str.substring(0,3) + “-” + num_str.substring(3,7);<br /> <br /> 515<br /> <br /> 15<br /> <br /> 516<br /> <br /> Day 15<br /> <br /> LISTING 15.6<br /> <br /> continued<br /> <br /> else if (num_str.length == 10) num_str = “(“ + num_str.substring(0,3) + “) “ + num_str.substring(3,6) + “-” + num_str.substring(6,10); else if (num_str_save.indexOf(“ INVALID!”) > -1) num_str = num_str_save; else num_str = num_str_save + “ INVALID!”; /* * DEBUG: display num_str in alert box */ textareaDebugWrite(“FORMATTED: num_str = “,num_str); /* * put formatted phone number back into the form */ obj.phoneNumber.value = num_str; } </script> </head> <body> <center> <h1>phoneNumberFormat</h1> <hr> <form name=”formInfo”> <b>Input Phone Number</b> <input type=”text” size=”30” name=”phoneNumber” onchange=”phoneNumberFormat(document.formInfo)”> </form> <hr> </center> <em>Jim O’Donnell, <a href=”mailto:jim@odonnell.org” rel="nofollow">jim@odonnell.org</a></em> <!-- DEBUG STUFF --> <hr> <form name=”debugInfo”> <textarea name=”debugText” rows=”8” cols=”70”>DEBUG INFO...

---

<script language=”javascript” type=”text/javascript”> function textareaDebugWrite(lbl,val) { document.debugInfo.debugText.value += “\n” + lbl + val; }

Debugging and Error Handling

OUTPUT FIGURE 15.14 Text-area boxes can be used to display debugging information.

After the script is executed, Figure 15.15 shows the resulting output. As shown, the telephone number has been correctly formatted. Additionally, the intermediate information that you can use to debug the script is shown in the text-area box. If the script does not work, this information would allow you to pinpoint where the problem exists.

OUTPUT FIGURE 15.15 All intermediate calculations can be output onto page.

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Note

Before the advent of Dynamic HTML, you could only use form elements, such as text and text-area boxes, to dynamically change text content once a Web page was loaded. HTML forms were among the first elements for Web pages accessible through JavaScript as well. Now, there are other ways to add and change content on a Web page using Dynamic HTML, and these techniques also can be used to include debugging information. However, using HTML forms is still the most straightforward approach.

Debugging Run-Time Errors II: Continuous Events Validation of HTML forms is a discrete event. When a form field is changed or a Submit button is clicked, a JavaScript runs once and performs its function (for example, validating and formatting the contents of a text box as in the example in the previous section). For this sort of script, the alert box and text-area methods shown in the previous section are appropriate. What about “continuous” events, such as those generated by mouse and keyboard events? Listing 15.7 shows an example of this, which is a positioning example that runs in Internet Explorer or Navigator 4. In this example, Dynamic HTML techniques are used to allow a Web-page graphic to be dragged anywhere on the page.

INPUT

LISTING 15.7

Dynamic HTML Allows Objects to Be Dynamically Moved (positioningExample.html)

<script language=”JavaScript” src=”dynlayer.js”> <script language=”JavaScript” src=”mouseevents.js”> <script language=”JavaScript” src=”drag.js”> <script language=”JavaScript”> function init() { /* * initialize DynLayers */ DynLayerInit(); /*

Debugging and Error Handling

LISTING 15.7

519

continued

* add the draggable layers to the drag object */ drag.add(author); /* * initialize mouse events */ initMouseEvents(); }

Positioning with
Cross-Browser Dynamic HTML

---

This example uses the DynAPI JavaScript Library ( http://www.dansteinman.com/dynduo/) to create an example that will work in Internet Explorer and Netscape Navigator 4.

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<em>Jim O’Donnell, [email protected]

Figure 15.16 shows the Web page, as it first appears when it is loaded. As shown in Figure 15.17, you can pick up and drag the image and move it around the Web page.

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OUTPUT FIGURE 15.16 JavaScript and Dynamic HTML can be used to move objects.

FIGURE 15.17 Continuous events need a way of displaying debugging information that will not interrupt them.

Debugging and Error Handling

Note

The Dynamic Duo Web site, located at http://www.dansteinman.com/ dynduo/, is a good example of what you can achieve using Dynamic HTML that is targeted for both Netscape Navigator and Microsoft Internet Explorer. The external JavaScripts loaded in this example are from this Web site. While this site is no longer being updated, it still has lots of good information, and links to more up-to-date content.

How do you debug scripts such as this? While the graphic is being dragged from one position to another, it is generating events continuously. Obviously, popping up an event box every time a script was called in this case would not be acceptable, as it would interrupt the dragging operation. Likewise, if a text-area box were used, as in the example in Listing 15.6, where information was appended to the box contents each time the script was called, the box would quickly fill up the text-area box and would not be useful. So, what is the answer? In this case, a good way to display intermediate or internal script values while it is continually running is to use an HTML form text box. (Another way would be to use the status bar area.) Instead of appending new content, the contents of the box or boxes are replaced each time the script is called. So, in the positioning example, each time a mouse event calls the script, it is able to display new information from the script internals. Listing 15.8 shows the example with debugging information included. In this case, a simple HTML table is used to format a collection of four text boxes, with each being used to show information about the object being dragged.

INPUT

LISTING 15.8 Text Boxes Can Be Updated On-The-Fly (positioningExampleDebug.html)

<script language=”JavaScript” src=”dynlayer.js”> <script language=”JavaScript” src=”mouseevents.js”> <script language=”JavaScript” src=”drag.js”> <script language=”JavaScript” type=”text/javascript”> function init() { /* * initialize DynLayers */ DynLayerInit(); /* * add the draggable layers to the drag object */

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LISTING 15.8

continued

drag.add(author); /* * set up supplemental event handlers */ drag.onDragStart = dragStart; drag.onDragMove = dragMove; drag.onDragEnd = dragEnd; /* * initialize mouse events */ initMouseEvents(); } function dragStart(x,y) { document.MyForm.ID.value = drag.obj.id document.MyForm.STATE.value = “moving” return false } function dragMove(x,y) { document.MyForm.X.value = drag.obj.x document.MyForm.Y.value = drag.obj.y return false } function dragEnd(x,y) { document.MyForm.STATE.value = “static” return false }

Positioning with
Cross-Browser Dynamic HTML

---

This example uses the DynAPI JavaScript Library ( http://www.dansteinman.com/dynduo/) to create an example that will work in Internet Explorer and Netscape Navigator 4.

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<em>Jim O’Donnell, [email protected]

Debugging and Error Handling

LISTING 15.8

523

continued

Drag Information	ID
	STATE
	X
	Y

Figure 15.18 shows this example when the page is loaded. As displayed, the debugging information can be found in the lower lefthand corner of the page. When the graphic is dragged with the mouse, as shown in Figure 15.19, the information in the text boxes is continuously updated to reflect the current position and state of the movement.

OUTPUT FIGURE 15.18 Text boxes are ideal for the display of changing data.

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FIGURE 15.19 Information associated with continuous events can be displayed as the events unfold.

Advanced Debugging Techniques In addition to the techniques shown above, which are designed to give you the ability to step through a script and look at some of the internal values and calculations, there are some other techniques that can be performed that are a bit more involved. In this section, a couple of these more advanced techniques will be briefly discussed. These techniques, when used with some of those described above, give you even more capabilities when debugging.

Creating a Run-Time JavaScript Interpreter Your JavaScript-enabled browser, whether it is Netscape Navigator, Internet Explorer, or even Opera, has a built-in JavaScript interpreter. When a Web page is loaded that includes JavaScript, these scripts are interpreted and the results are output or displayed, or in some way acted upon by the Web browser. In essence, this is a “batch” process— where you write a bunch of JavaScript code and submit it to the browser to see if it runs. In some cases, it would be nice to have the equivalent of a command-line interface to the JavaScript browser. This would allow you to type in a line of JavaScript and have it immediately evaluated and its output displayed. Fortunately, JavaScript has a function that gives you this ability.

Debugging and Error Handling

Listing 15.9 shows a JavaScript file that implements a simple JavaScript interpreter using the prompt() method. When loaded and called from another JavaScript, a prompt box will display, which takes a line of JavaScript input, evaluates it, and shows the results. LISTING 15.9

INPUT

JavaScript eval()Function Used as JavaScript Interpreter (debugInterpreter.js)

function debugInterpreter () { var pstr = “JAVASCRIPT INTERPRETER”; while (true) { x = prompt(pstr); if (!x) break; pstr = “eval(“ + x + “) = “ + eval(x); } }

The JavaScript interpreter shown in Listing 15.10 uses the JavaScript eval() function to interpret JavaScript expressions entered into the prompt box. It then redisplays a prompt box with the results of the previous line while ready to accept a new line of input. Clicking on the Cancel button of the prompt box exits the interpreter.

ANALYSIS

Listing 15.10 shows an application of the interpreter, as it is included in the telephonenumber formatting example. By writing the interpreter as a separate function and calling it, as in Listing 15.10, you preserve the modularity of the code, thereby making it very simple to include in a script. The trade-off for doing it this way is that, while in the interpreter, you can access only JavaScript variables and browser properties that have global scope. You will notice in Listing 15.10 that a few of the variables have been made global by declaring them outside of the JavaScript function. Alternatively, you could actually include the debug interpreter while loop itself in your script, rather than as a separate function; this is a little messier, but gives access to all of the local variables in the script.

INPUT

LISTING 15.10

A Real-Time Interpreter Can Be Used for Debugging (phoneExampleDebug3.html)

<script language=”javascript” src=”debugInterpreter.js”> <script language=”javascript” type=”javascript”> var num_str,num_str_save; function phoneNumberFormat(obj) { var xChars = new Array(“ “,”(“,”)”,”+”,”-”,”.”, “I”,”N”,”V”,”A”,”L”,”D”,”!”);

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LISTING 15.10

continued

/* * remove all extraneous characters leaving only numbers */ num_str = obj.phoneNumber.value; num_str_save = num_str; for (i = 0;i < xChars.length;i++) { x = xChars[i]; while (num_str.indexOf(x) > -1) num_str = num_str.substring(0,num_str.indexOf(x)) + num_str.substring(num_str.indexOf(x) + 1,num_str.length) } /* * remove leading 1, if present */ if (num_str.indexOf(“1”) == 0) num_str = num_str.substring(1,num_str.length); /* * verify length of number, if valid */ if (num_str.length == 7) num_str = num_str.substring(0,3) + “-” + num_str.substring(3,7); else if (num_str.length == 10) num_str = “(“ + num_str.substring(0,3) + “) “ + num_str.substring(3,6) + “-” + num_str.substring(6,10); else if (num_str_save.indexOf(“ INVALID!”) > -1) num_str = num_str_save; else num_str = num_str_save + “ INVALID!”; /* * DEBUG: call debug interpreter */ debugInterpreter(); /* * put formatted phone number back into the form */ obj.phoneNumber.value = num_str; }

phoneNumberFormat

---

Input Phone Number

Debugging and Error Handling

LISTING 15.10

527

continued

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<em>Jim O’Donnell, [email protected]

Figure 15.20 shows an example of the interpreter in action. It has already been used to print out the current contents of the num_str variable, and it is in the process of showing the contents of the HTML form text box.

OUTPUT FIGURE 15.20 A simple script can be used as a realtime JavaScript interpreter.

Opening a Debug Browser Window Another alternative that can be used to the text-area method shown in an earlier example is to use JavaScript to open a completely new window and to direct debug output into the new window. By opening a new window, you can use the document.writeln() method to write debug output into it. New windows can be opened using the document.open() method.

Using JavaScript Debuggers In addition to the techniques shown in this chapter, there are a few tools available for debugging JavaScript. Both Microsoft and Netscape have JavaScript debuggers available. The Microsoft debugger is available at http://msdn.microsoft.com/scripting/debugger/

The Netscape debugger can be downloaded from http://developer.netscape.com/software/jsdebug.html

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In this chapter, we are not going to go into any great depth into the operations of these debuggers. Most of the capabilities that they give are similar to the ones that were added to our own scripts using the techniques above. Each of the debuggers has its own strengths, weaknesses, and quirks, and will not be discussed fully. Instead, some of the capabilities of the debuggers will be shown with illustrations from Microsoft’s Script Debugger. Some of the capabilities that are present in the script debuggers are as follows: • Entering a script upon error—Internet Explorer can be set up, using one of the check boxes under the Advanced tab of Internet Options, to enter the script debugger upon script errors. If this is done, then a script error will pop up an alert box similar to the one shown in Figure 15.21. One of the quirks of the script debugger is that you have to click on OK to enter the debugger; if you click on Cancel, the alert box will continue to pop up. • Stepping through a script—Once you have entered a script, the script debugger enables you to proceed through it in a variety of ways. You can single step through it, or run until the next breakpoint (see below). As shown in Figure 15.22, Microsoft’s Script Debugger indicates the current line with a yellow arrow along the left-hand side. • Setting breakpoints—A breakpoint is something you can set within a program that causes the debugger to halt whenever it reaches that spot. In the case of a JavaScript, you might want to set a breakpoint just before a crucial operation, thus allowing yourself to use the debugger to check the values of the information used in that operation. In the Script Debugger, a breakpoint is denoted with a small stopsign icon along the left side. • Entering JavaScript commands—If a script debugger allows you to stop the script at a given point (or before each statement), in order for it to be useful you need to be able to get further information on the current state of the script. You can accomplish this by allowing JavaScript statements and expressions to be inputted and immediately evaluated. The small window in Figure 15.22 shows the Script Debugger command window; every other line in the window was typed in directly, followed by what it evaluated. FIGURE 15.21 JavaScript errors give you the option to enter the debugger.

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FIGURE 15.22 The script debugger gives many options to fix script errors.

Summary As with programming in any other language, the process of programming in JavaScript often involves spending some time tracking down and fixing bugs in what you have written. Both Microsoft and Netscape have developed script debuggers which ideally would make this job easier, but neither of their offerings is terribly robust. This chapter has shown you a variety of ways that you can use some of JavaScript’s own language elements to carefully step through your code and narrow down the location and cause of any errors.

Workshop In this workshop we will review some of the debugging techniques you have learned in this chapter.

Q&A Q. Where are two places you can display debugging information within a web page while other events are taking place? A. Either in HTML form fields, normally text or text-area boxes, or in the status area of the browser window.

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Q. Why aren’t alert boxes appropriate for debugging JavaScripts that involve continuous events? A. By their very nature, an alert box interrupts anything else that is happening in a web page or JavaScript. As soon as the continuous event began, such as one involving mouse movement or dragging, the alert box would pop up and stop it.

Quiz 1. If you had a JavaScript that looked at a preexisting cookie that gave a “last visited” time, in order to print out how long it had been since the last time the user accessed a given Web page, what type of script would that be? 2. What use could a confirm box (as opposed to the alert and prompt box uses shown above) be put to when debugging a JavaScript?

Quiz Answers 1. That sort of script would be a load-time script, using the document.cookie property. 2. An alert box allows you to display information and pause before continuing. With a little more logic, you could use a confirm box instead, but give the option to either continue or abort the script.

Exercise Use document.open() and document.writeln() to create a separate debug window for information from a run-time, discrete event script.

WEEK 3

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16

Cookies: Storing Persistent Data Most Web servers have very short memories. When you request a page, the server usually doesn’t really know who you are, what you entered on a form three pages ago, or whether this is your first visit to the site or your 75th visit. One of the challenges of using the Hypertext Transfer Protocol (HTTP) is that it doesn’t track the state of your interactions with the server. State refers to any information about you or your visit to a Web site. It can be maintained as you move from page to page within the site, and it can be used by the Web server or a JavaScript program (or both) to customize your experience at the site. But if HTTP doesn’t maintain the state, what does? This chapter shows you how to get around HTTP’s limitations by using cookies, which allow you to store persistent data about someone’s visit to your web site. In addition to the material on cookies, you will also be introduced to a few other methods of preserving information from one Web page to the next, using URL query string parameters and hidden form variables.

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This chapter will teach you • How and why you would want to maintain state information • What cookies are and how they work • How to use cookies in your Web pages • How to use other state maintenance options

Maintaining State Maintaining state means remembering information while the user moves from page to page within a Web site. With this information at hand, you can set user preferences, fill in default form values, track visit counts, and do many other things that make browsing easier for users and that give you more information about how your pages are used. You can maintain state information in a number of ways: • Store it in cookies • Encode it in URL links • Send it in hidden form variables • Store it in variables in other frames • Store it on the Web server Be aware, however, that some technical challenges regarding state maintenance can occur. While browsing a site, a user might suddenly zoom off to another Web site and return minutes, hours, or days later, only to find that any saved state information is out of date or has been erased. He or she might return by clicking the browser’s Back button, by using a bookmark, or by typing in the URL directly, causing state information encoded in the URL to be overwritten or lost. The Web developer must maintain state information regardless of whether the user navigates through the site using buttons on a form or a URL link on a page. This could mean adding information to both hidden form variables and every URL tag that appears on the page. With all these difficulties to overcome, these state maintenance mechanisms had better be useful. Luckily, they are. Many advantages exist to maintaining state both within a single site visit and from one visit to the next. Consider the following scenarios: • A shopping cart application—Users could browse through the site while selecting and adding items to a virtual shopping cart. At any time, they can view the items in the cart, change the contents of their cart, or take the cart to the checkout counter for purchase. Keeping track of which user owns which shopping cart is essential.

Cookies: Storing Persistent Data

• Custom home pages—Many Web sites have now set up home pages where users can customize what they see when they arrive. After giving the user a choice of layouts, color schemes, and favorite destinations, it stores the preferences on the user’s own computer through the use of cookies. Some sites ask for your zip code and store this information in a cookie, and use it to provide you with local weather and news information. The user can return to the site any time and get the previously configured page. • Frequent visitor bonuses—By storing information on the client computer, this application keeps track of how many times a browser has hit a particular page. When the user reaches a certain level of hits, he or she gets access to more or better services. • Change banners—You can make the graphical banners and the text change each time the user hits a page. This technique is often used to cycle through a list of advertisements. • Bookmarks—Remember where a user was when he last visited the site. Was he reading a story, filling out a questionnaire, or playing a game? Let him pick up where he left off. • Login information—If your site requires users to log on, you can give the user the option of allowing your Web site to remember his or her username and/or password. This information can be stored in a cookie and therefore allow the user faster access to your site in the future. • Games—Remember current or high scores, and hence present new challenges based on past answers and performance.

Cookies: An Introduction Cookies—sometimes called magic cookies, but more formally known as persistent clientstate HTTP cookies—enable you to store information on the client browser’s computer for later retrieval. Although they have their drawbacks, cookies are the most powerful technique available for maintaining state within a Web site. Netscape came up with the original cookie specification. There doesn’t seem to be any good reason why Netscape chose that particular name. In fact, on its cookie specification page, Netscape admits “the state object is called a cookie for no compelling reason.” In their simplest form, cookies store data in the form of name=value pairs. You, the developer, can pick any name and value combination you want. More advanced cookie features include the capability to set an expiration date and to specify which Web pages can see the cookie information.

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Advantages of Cookies One of the most powerful aspects of cookies is its persistence. When a cookie is set on the user’s browser, it can persist for days, months, or even years. This makes it easy to save user preferences and visit information, and to keep this information available every time the user returns to your site. Cookies prove especially helpful when used in conjunction with JavaScript. Because JavaScript has functions for reading, adding, and editing cookies, your JavaScript programs can use them to store global information about a user as he surfs through your Web site.

Limitations of Cookies Some limitations of cookies could prove problematic. Cookies are stored on the user’s computer, usually in a special cookie file or, as with Internet Explorer, in a number of files. As with all files, this cookie file might be accidentally (or purposely) deleted, taking all the browser’s cookie information with it. The cookie file could be write protected, thus preventing any cookies from being stored there. Browser software could impose limitations on the size and number of cookies that can be stored, and newer cookies might overwrite older ones. Because cookies are associated with a particular browser, problems arise if users switch from one browser to another. If you usually use Netscape Navigator and have a collection of cookies, they will no longer be available for you to use if you decide to switch to Microsoft Internet Explorer. Finally, if several people use the same computer and browser, they could find themselves using cookies that belong to someone else. The reason for this is that cookie information is stored in a file on the computer, and depending on how the computer is set up, the browser might have no way to distinguish between multiple users.

Disadvantages of Cookies Some problems, both real and imagined, also occur with the use of cookies. Because many browsers store their cookie information in an unencrypted text file, you should never store sensitive information, such as a password, in a cookie. Anyone with access to the user’s computer could read it. Netscape Navigator and Microsoft Internet Explorer browsers have a feature that can alert the user every time an attempt is made to set a cookie. These browsers can even be configured to prevent cookies from being set at all. This sometimes results in confusion on the user’s part when a dialog box informs her that something strange involving a cookie is happening to her computer. If cookies are disabled, your carefully designed

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Web application might not run at all, unless you are careful to not rely on the cookie information being there.

Cookie Myths The biggest problem facing cookies could be a psychological one. Some savvy Web users believe that cookies are used by “Big Brother” as a tool to violate their privacy. Considering the capabilities of cookies for storing information about where specifically they have visited on a Web site, how many times they have been there, which advertising banners they have viewed, and what they have selected and placed on forms, some people think their privacy is invaded whenever a cookie gets set on their computer. In reality, cookies are seldom used for these purposes, although technically these things are possible. The most common use of cookies is to give developers an easy way to customize their Web sites for everyone that visits them. A site can ask you for your name or ask you about some other preference once, and then store this information in a cookie. That way, the next time you visit the site, it doesn’t have to ask you for this same information again, assuming you are using the same browser. Other users complain about Web sites that write information to their computers and take up space on their hard drives. This is somewhat true. Web browser software limits the total size of the cookies stored, as well as the amount of space that can go to the cookies of a particular Web site. Consider, however, that this limit probably is small when compared to the size of the pages and graphic images that Web browsers routinely store in their page caches. Other users are concerned that cookies set by one Web site might be read by other sites. This is completely untrue. Your Web browser software prevents this from taking place by making cookies available only to the sites that created them. If your users understand the usefulness of cookies, this “cookie backlash” shouldn’t be a problem. As mentioned previously, Netscape came up with the original cookie specification. You can find more information about cookies on the Netscape Web site at http://www.netscape.com/newsref/std/cookie_spec.html

Using Cookies By now you have considered the pros and cons of cookies and have decided that they are just what you need to make your JavaScript application a success.

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This section discusses a number of handy functions for reading and setting cookies, which will help you make your Web sites smarter and more user-friendly. Also included are Internet references for finding additional information concerning cookies.

Retrieving Cookie Values Cookie names and values are stored and set using the cookie property of the document object. To store the raw cookie string in a variable, you would use a JavaScript command such as the following: var myCookie = document.cookie;

To display it on a Web page, use the following command: document.write(“Raw Cookies: “ + document.cookie + “
”);

JavaScript stores cookies in the following format: name1=value1; name2=value2; name3=value3

Individual name=value pairs are separated by a semicolon and a blank space. No semicolon is used after the final value. To retrieve a particular cookie, you can use a JavaScript routine such as the one shown in Listing 16.1.

INPUT

LISTING 16.1

JavaScript Function for Retrieving a Specific Cookie,

favoritesList.htm (excerpted)

// // GetCookie - Returns the value of the specified cookie or null // if the cookie doesn’t exist // function GetCookie(name) { var result = null; var myCookie = “ “ + document.cookie + “;”; var searchName = “ “ + name + “=”; var startOfCookie = myCookie.indexOf(searchName) var endOfCookie; if (startOfCookie != -1) { startOfCookie += searchName.length; // skip past cookie name endOfCookie = myCookie.indexOf(“;”,startOfCookie); result = unescape(myCookie.substring(startOfCookie,endOfCookie)); } return result; }

Cookies: Storing Persistent Data

Note

Most of the listings that appear in this chapter are excerpts from the favoritesList.htm document discussed in the “A Cookie Example” section later in this chapter.

In Listing 16.1, the myCookie string is created with a leading space and trailing semicolon; this helps cookie processing by making sure all cookie string names start and end similarly. From there, it is easy to find the start of the name= portion of the string, skip it, and retrieve everything from that point until the next semicolon.

ANALYSIS

Setting Cookie Values The name=value combination is the minimum amount of information you need to set up a cookie. However, there can be more to cookies than just this. The complete list of parameters, which should be separated by a space and semicolon, that you can use to specify a cookie is as follows: •

name=value

•

expires=date

•

path=path

•

domain=domainname

•

secure

Choosing Meaningful Cookie Names and Values The name and value can be anything you choose. In some cases, you might want it to be very explanatory, such as FavoriteColor=Blue. In other cases, it could just be code that the JavaScript program interprets, such as CurStat=1:2:1:0:0:1:0:3:1:1. In any case, the name and value are completely up to you. Listing 16.2 shows the simplest way to create cookies. The function SetCookieEZ() is a routine to add a single name=value pair to a cookie.

INPUT

537

LISTING 16.2

Adding Cookies Is Easy with JavaScript, favoritesList.htm

(excerpted)

// // SetCookieEZ - Quickly sets a cookie which will last until the // user shuts down his browser // function SetCookieEZ(name,value) { document.cookie = name + “=” + escape(value); }

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Notice that the value is encoded using the JavaScript escape() function. If there were a semicolon in the value string itself, it might prevent you from achieving the expected results. Using the escape() function eliminates this problem.

ANALYSIS

Also notice that the document.cookie property works rather differently from most other properties. In most other cases, using the assignment operator (=) causes the existing property value to be completely overwritten with the new value. This is not the case with the cookie property. With cookies, each new name you assign is added to the active list of cookies. If you assign the same name twice, the second assignment replaces the first. Some exceptions exist to this last statement; these are explained in the “Path” section later in this chapter.

Setting a Cookie Expiration Date The expires=date property tells the browser when the cookie will expire. The cookie specification page at Netscape states that dates are in the form of Wdy, DD-Mon-YY HH:MM:SS GMT

Here’s an example: Mon, 08-Jul-96 03:18:20 GMT

This date format is based on Internet RFC 822, which you can find at http://www.w3.org/hypertext/WWW/Protocols/rfc822/#z28. The only difference between RFC 822 and the Netscape implementation is that in Netscape Navigator, the expiration date must end with GMT (Greenwich Mean Time). The JavaScript language provides a function to do just that. By using the toGMTString() function, you can set cookies to expire in the near or distant future

Tip

Even though the date produced by the toGMTString() function doesn’t match the Netscape specification, it still works under JavaScript.

If the expiration date isn’t specified, the cookie remains in effect until the browser is shut down. The following is a code segment that sets a cookie to expire in one week (where the number of milliseconds in one week equals 7 days/week × 24 hours/day × 60 minutes/hour × 60 seconds/minute × 1000 milliseconds/second):

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var name=”foo”; var value=”bar”; var oneWeek = 7 * 24 * 60 * 60 * 1000; var expDate = new Date(); expDate.setTime(expDate.getTime() + oneWeek); document.cookie = name + “=” + escape(value) + “; expires=” + expDate.toGMTString();

How to Delete a Cookie To delete a cookie, set the expiration date to some time in the past—how far in the past doesn’t generally matter. To be on the safe side, a few days prior should work fine. The following is a routine to delete a cookie, shown in Listing 16.3.

INPUT

LISTING 16.3

Use the Cookie Expiration Date to Delete an Unwanted Cookie, favoritesList.htm (excerpted)

// // ClearCookie - Removes a cookie by setting an expiration date // three days in the past // function ClearCookie(name) { var ThreeDays = 3 * 24 * 60 * 60 * 1000; var expDate = new Date(); expDate.setTime(expDate.getTime() - ThreeDays); document.cookie = name + “=NA; expires=” + expDate.toGMTString(); }

When deleting cookies, it doesn’t matter what you use for the cookie value—any value will do.

Caution

Some versions of Netscape do a poor job of converting times to GMT. Some common JavaScript functions for deleting a cookie consider the past to be 1 millisecond behind the current time. Although this is usually true, it doesn’t work on all platforms. To be on the safe side, use a few days in the past to set the expiration for the cookies.

Using the Cookie Path to Adjust Its Accessibility By default, cookies are available to other Web pages within the same directory as the page on which they were created. The path parameter enables a cookie to be made available to pages in other directories. If the value of the path parameter is a sub-string of a

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page’s URL, cookies created with that path are available to that page. You could create a cookie, for example, with the following command: document.cookie = “foo=bar1; path=/javascript”;

This would make the cookie foo available to every page in the javascript directory and all those directories beneath it. If, instead, the command looked like this: document.cookie = “foo=bar2; path=/javascript/sam”;

the cookie would be available to sample1.html, sample2.html, sammy.exe, and so on, if they are located in the javascript directory or one of its subdirectories. Finally, to make the cookie available to everyone on your server, use the following command: document.cookie = “foo=bar3; path=/”;

What happens when a browser has multiple cookies on different paths but with the same name? Which one wins? Actually, they all do. When this situation arises, it is possible to have two or more cookies with the same name but with different values. If a page issued all the commands listed previously, for example, its cookie string would look like the following: foo=bar3; foo=bar2; foo=bar1

To help be aware of this situation, you might want to write a routine to count the number of cookie values associated with a cookie name. It would look something like this: function GetCookieCount(name) { var result = 0; var myCookie = “ “ + document.cookie + “;”; var searchName = “ “ + name + “=”; var nameLength = searchName.length; var startOfCookie = myCookie.indexOf(searchName) while (startOfCookie != -1) { result += 1; startOfCookie = myCookie.indexOf(searchName,startOfCookie + nameLength); } return result; }

Of course, if a GetCookieCount function exists, a GetCookieNum function should be available to retrieve a particular instance of a cookie. That function would look like this:

Cookies: Storing Persistent Data

function GetCookieNum(name,cookieNum) { var result = null; if (cookieNum >= 1) { var myCookie = “ “ + document.cookie + “;”; var searchName = “ “ + name + “=”; var nameLength = searchName.length; var startOfCookie = myCookie.indexOf(searchName); var cntr = 0; for (cntr = 1; cntr < cookieNum; cntr++) startOfCookie = myCookie.indexOf(searchName, startOfCookie + nameLength); if (startOfCookie != -1) { startOfCookie += nameLength; // skip past cookie name var endOfCookie = myCookie.indexOf(“;”,startOfCookie); result = unescape(myCookie.substring(startOfCookie,endOfCookie)); } } return result; }

To delete a cookie, the name and the path must match the original name and path used when the cookie was set.

Using the Cookie Domain to Adjust Its Accessibility Usually, after a page on a particular server creates a cookie, that cookie is accessible only to other pages on that server. Just as the path parameter makes a cookie available outside its home path, the domain parameter makes it available to other Web servers at the same site, as described in the next paragraph. You can’t create a cookie that anyone on the Internet can see. You can only set a path that falls inside your own domain. This is because the use of the domain parameter dictates that you must use at least two periods (for example, .mydomain.com) if your domain ends in .com, .edu, .net, .org, .gov, .mil, .int, or the newer root domains such as .info. Otherwise, if the domain ends in a two-letter country code, it must have at least three periods (.mydomain.ma.us). Your domain parameter string must match the tail of your server’s domain name.

How to Use the Cookie secure Parameter The final cookie parameter tells your browser that this cookie should be sent only under a secure connection with the Web server. This means that the server and the browser must support HTTPS security. (HTTPS is Netscape’s Secure Socket Layer Web page encryption protocol.) If the secure parameter is not present, cookies are sent unencrypted over the network.

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Note

You can’t set an infinite number of cookies on every Web browser that visits your site. The following list shows the number of cookies you can set and how large they can be: • Cookies per server or domain: 20 • Total cookies per browser: 300 • Largest cookie: 4KB (including both the name and value parameters If these limits are exceeded, the browser might attempt to discard older cookies by tossing out the oldest cookies first.

Now that you have seen all the cookie parameters, it would be helpful to have a JavaScript routine set cookies with all the parameters. The SetCookie() function shown in Listing 16.4 does just that.

INPUT

LISTING 16.4 JavaScript Routine to Add a Cookie, Including Any Optional Parameters, favoritesList.htm (excerpted)

// // SetCookie - Adds or replaces a cookie. Use null for parameters // that you don’t care about // function SetCookie(name,value,expires,path,domain,secure) { var expString = ((expires == null) ? “” : (“; expires=” + expires.toGMTString())) var pathString = ((path == null) ? “” : (“; path=” + path)) var domainString = ((domain == null) ? “” : (“; domain=” + domain)) var secureString = ((secure == true) ? “; secure” : “”) document.cookie = name + “=” + escape(value) + expString + pathString + domainString + secureString; }

To use this routine, you call it with whatever parameters you care about and use null in place of parameters that don’t matter.

A Cookie Example The JavaScript program in this example is in the HTML document favoritesList.htm. Excerpts of the program were shown in Listings 16.1 through 16.4; these showed the JavaScript functions used to create and manipulate the document cookies used in this example. The full listing of the document is shown in Listing 16.5, which enables the

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user to create a personalized Web page containing links to sites of general interest in a number of categories. The user’s favorite links are stored in cookies.

INPUT

LISTING 16.5 The Section of the Cookie Example, favoritesList.htm (excerpted)

<script type=”text/javascript” language=”javascript”> <script type=”text/javascript” language=”javascript”>

---

Current Document Cookie Contents...

<script type=”text/javascript” language=”javascript”>

---

<em>Jim O’Donnell, [email protected]

As shown in Listing 16.5, when this page is loaded, one of two JavaScripts is called to actually “fill” the page: either SendOptionsPage() or SendPersonalPage(). The former enables the user to select from a list of sites to be included as favorites; the latter is used to display those sites (or to display all the possible sites). Figure 16.1 shows this page when it first loads, before the user has selected a list of favorites (so all possible sites are shown).

ANALYSIS

OUTPUT FIGURE 16.1 The Favorites page displays all possible sites when first loaded.

Before JavaScript, a task such as this would have been handled at the server level. Each hit would have involved having the server run some type of script or program to read the user’s cookies and generate his page on-the-fly. With JavaScript, all this processing takes place in the client’s browser. The server just downloads the static page—and it might not even need to do that because the page might come from the client’s local cache.

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When the page loads, all the links, selected or not, are sent. The client, with the help of cookies and JavaScript, decides which ones to show the user. This program makes use of three cookies: The Favorites cookie contains a unique code for each favored link. The ViewAll cookie toggles between showing the user’s favorites and all possible links. The program can also display either of two pages: one for displaying the selected links, and the other for changing the configuration and options. When the ShowOptions cookie is set, the Options selection page is displayed. Otherwise, the regular page is shown. When the screen shown in Figure 16.1 is displayed after the page loads for the very first time, the document cookie has not been set yet, so it is empty, which results in all links displaying. If View Favorites is clicked at this point, then the document cookie is set to ViewAll=F

and the screen shown in Figure 16.2 is displayed—empty, because no favorites have been selected yet. Clicking the Select Personal Favorites button gets the screen shown in Figure 16.3, where favorites can be selected from the list of choices. One such selection might result in the Favorites list shown in Figure 16.4, which has the document cookie value of ViewAll=F; Favorites=null%3Ccfbofw%3E%3Csyah%3E%3Cmjod%3E

which indicates a selection of the “For Better or For Worse”, “Yahoo”, and my home page as favorites.

Note

The %XX encoding, such as the %3C and %3E shown above, is used for such symbols as semicolons, commas, and whitespace.

Note

You might notice in Figures 16.2 and 16.4 that the current contents of the document cookie are displayed in a text-area box at the bottom of the page. This is done in this example so you can see the changes to the cookie as they occur; in an actual “production” page, you wouldn’t include it. The Clear Cookies button shown in these two figures is also something you probably wouldn’t include in the final page.

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OUTPUT FIGURE 16.2 An empty Favorites list doesn’t yield a very exciting Web page.

FIGURE 16.3 The Select Favorites page displays all the possible sites as checkboxes and enables the user to select and deselect which to use as favorites.

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FIGURE 16.4 By enabling users to personalize their copy of your Web page, you allow a more personal experience without a greater burden on your server.

Once this list of favorites is created, any of them can be selected to load the corresponding Web page, as shown in Figure 16.5. FIGURE 16.5 By using cookies to customize your site for your users, you can allow them to navigate through it more easily.

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The program creates objects called favorites. Each favorite is, in essence, a Web link to another page. The favorite contains information on the link’s URL, a user-friendly page description, and the code that identifies it in the Favorites cookie string. The favorite also knows how to print itself on a Web page as a regular link for the Favorites page, or in a checkbox format for the Options page. The functions used to manipulate the cookies in the favoritesList.htm example were shown in Listings 16.1 through 16.5. The other functions used in this example are summarized in the following list: •

SendOptionsPage()—Loads the Web browser with a page that enables the user to select which sites to be included as favorites.

•

SendPersonalPage()—Loads

•

WriteAsCheckBox()—Used

•

WriteAsWebLink()—Used

the Web browser with a page that shows either the user’s favorites or all the sites as hypertext links.

by SendOptionsPage to display each potential favorite site as a check box, to enable the user to select or deselect it. by SendPersonalPage to display each site as a

hypertext link. •

LoadOptions()—This

function is called to initiate the display of the options page.

•

ToggleView()—This

•

favorite()—This

function is used to create a JavaScript object that is used to store the information used to define a favorite site.

•

Enabled()—This

•

Checked()—This

•

isEnabled()—Returns true

function is called to toggle the personal page between displaying favorites and all sites.

JavaScript function is used as a method by the favorite object; it returns true if the link corresponding to this object is enabled. JavaScript function is used as a method by the favorite object; it returns the string CHECKED if the link corresponding to this object is enabled. if the favorite identified by the Name parameter passed

to the function is enabled. •

AddFavorite()—Enables

the favorite identified by the Name parameter passed to

the function. •

ClearFavorite()—Disables

the favorite identified by the Name parameter passed

to the function. •

ClearCookies()—Clears

the three cookies associated with this page.

•

SetFavoriteEnabled()—Enables

•

ReloadPage()—Reloads

or disables the favorite identified by the Name parameter passed to the function by calling AddFavorite or ClearFavorite. the Web browser with the current page; what is displayed, however, changes according to the current state of the Document cookie.

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Where Are Cookies Going? As mentioned earlier, cookies were designed and first implemented by Netscape. However, the Internet Engineering Task Force (IETF) has a committee—the Hypertext Transfer Protocol (HTTP) Working Group—whose charter it is to examine, document, and suggest ways to improve HTTP. You can find a link to the HTTP Working Group’s latest Internet Draft, called “HTTP State Management Mechanism,” at http://www.ietf.cnri.reston.va.us/rfc/rfc2109.txt. Although the specification resembles Netscape cookies in theory, if not in syntax, it does have a few notable differences. It doesn’t encourage having cookies around much longer than the browser session. If the new specification is accepted, cookies will be given a Max-Age lifetime rather than an expires date. All cookies still expire when their time comes. Reading the specification provides insight into the complexities that surround the inner workings of cookies; it is well worth the read, regardless of whether the specification is approved.

Where to Find More Information About Cookies Although other ways of Web programming, such as CGI and special server interfaces, require that the server as well as the browser understand cookies, only the browser matters to JavaScript. This means, in general, that you can use JavaScript with impunity as long as you know your clients are JavaScript-capable, and they can run them with JavaScript enabled. Many JavaScript Web applications probably mix the language with other development tools, however, which would require the server to understand cookies. Nowadays, though, most clients and servers in use have both JavaScript and cookie support. You can find cookie information at the following locations on the Web: • Netscape cookie spec page (referenced previously in this chapter): http://www.netscape.com/newsref/std/cookie_spec.html

• Browsers supporting cookies: http://www.research.digital.com/nsl/formtest/stats-bytest/NetscapeCookie.html

• Cookie Central: http://www.cookiecentral.com/

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• Robert Brooks’ Cookie Taste Test: http://www.geocities.com/SoHo/4535/cookie.html

• Article about tracking cookies from the HotWired Web site: http://www.arctic.org/~dgaudet/cookies

Other State Maintenance Options As mentioned earlier in this chapter, a few drawbacks exist to using cookies. Perhaps you would rather just avoid the controversy and find some other way to maintain state from one page to the next. Two ways of doing this are available. Which one you use depends on how you, the developer, will have your users get from one page to the next. The main limitation of these methods is that they work only from one page to the page immediately following the previous page. If state information is to be maintained throughout a series of pages, these mechanisms must be used on every single page.

Using Query Strings to Send Information If most of your navigation is done through hypertext links embedded in your pages, you can add extra information to the end of the URL. This is usually done by adding a question mark (?) to the end of your Web page URL, followed by information in an encoded form, such as that returned by the escape method. To separate one piece of information from another, place an ampersand (&) between them. If you want to send the parameters color=blue and size=extra link, for example, you could use a link such as this:

large

along with your

XL Blue

This format is the same as the format used when submitting forms using the get method. A succeeding page can read this information by using the search property of the location object. This property is called search because many Internet search engines use this part of the URL to store their search criteria. The following is an example of how to use the location.search property. In this example, the name of the current page is sent as a parameter in a link to another page. The other page reads this property through the search property and states where the browser came from. Listing 16.6 shows the first page that contains the link.

Cookies: Storing Persistent Data

INPUT

LISTING 16.6

You Can Include Extra Parameters in the href to Pass State Information, whereOne.htm

Where Was I? (Page 1)

---

This page sets information which will allow the page to which it is linked to figure out where it came from. It uses values embedded in the link URL in order to do this.

We’ll assume that any URL parameters are separated by an ampersand.

Notice that there doesn’t need to be any JavaScript code in this page.

And now... ON TO PAGE 2!!!

---

<em>Jim O’Donnell, [email protected]

Listing 16.7 shows the second page, which demonstrates how to use location.search to find where the browser came from.

INPUT

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LISTING 16.7

Access href Information Using the window.location.search Property, whereTwo.htm

Where Was I? (Page 2)

---

This page reads information which allows it to figure out where it came from.

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LISTING 16.7

continued

<script type=”text/javascript” language=”JavaScript”>

---

<em>Jim O’Donnell, [email protected]

Figures 16.6 and 16.7 show the two Web pages, demonstrating that the first was able to pass information to the second.

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OUTPUT FIGURE 16.6 Extra information can be included in a hypertext link using the ? character.

FIGURE 16.7 By including extra information in your hypertext links, you can enable some state information to be passed among pages in your Web site.

Note that the above techniques can also be used to send information from one Web page to another with query strings by using the # instead of the ? character. The method is the same, except that you use location.hash instead of location.search to look through the string.

How to Use Hidden Form Variables The method used in the preceding section works fine as long as the user navigates from one page to another using links. To do the same thing with forms, you can use hidden form variables rather than the location.search parameter. Hidden form variables have the following format:

You can specify whatever you like instead of HiddenFieldName and HiddenFieldValue for the values of the name and value attributes of the element.

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Using hidden fields does not necessarily require the use of JavaScript code. They are defined, instead, in the tag of normal HTML documents. You normally will need to have a server-based script, such as a CGI program or a server API program, to read the values of these hidden fields. The form containing the hidden variables is submitted to a server script, which can then process the information for subsequent pages. It is possible to avoid the need for server processing of hidden form fields if you include them in a hidden frame, as well. At this point, however—using hidden form fields in hidden frames—you’re better off going ahead and using cookies.

Summary It is often useful for a Web page to remember information about its different users from one visit to another. This information can be collected and maintained on the server, but an easier way to remember a few things is through the use of browser cookies. Cookies are stored on your users’ computers, and allow your Web pages to remember information about them from one visit to another, which can enable you to customize and improve their experience on your page.

Workshop In this workshop we will review what you have learned about cookies in today’s chapter.

Q&A Q. How can you set up a cookie so that it can be accessed from Web pages on other parts of your Web site? A. By setting the cookie Path and/or Domain parameters, the cookies can still be accessed from other locations within your domain; but they still are not accessible from other domains. Q. Can you set a cookie equal to more than one value? A. Yes, by using a character such as an ampersand to append separate cookie values, and then by using JavaScript’s string functions to split the string apart.

Quiz 1. What are the two ways to clear a cookie that has been set with no expiration date? 2. In addition to using the location.search property, what is one other way of implementing an URL-based, state maintenance technique?

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Quiz Answers 1. Within JavaScript, the cookie can be cleared by setting a cookie with the same name with an expiration date in the past. Another way to clear a cookie set with no expiration date is for the user to close the browser. 2. By using the location.hash property. Just as location.search is set equal to the part of a URL after the ?, location.hash is set equal to the part after the hash mark symbol #.

Exercise Create a Web page with a cookie that keeps track of the amount of time since the last time a user viewed the page, and prints the time out at the bottom of each page. Here’s a tip: To make this time as accurate as possible, you can use the onUnload event to set the last viewed time when the page is exited.

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17

Privacy and Security This chapter will look at the privacy and security issues you could face with visitors to your Web page and dealing with JavaScript code. Internet security issues have become a hot topic these days, and Web users have become more leery about where they visit and the type of information they provide to those sites they do visit. By the same token, corporations have had to become more vigilant in their efforts to secure the information on their own corporate sites. Quite often the information provided on a Web page is not intended for all potential visitors. Because of this, you need to look at methods to secure the content, such as password protection. This chapter will teach you: • Privacy for Web users • Privacy for Webmasters • The impact on JavaScript

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Privacy for Web Users Security issues for Web users have become a major concern—everyone is concerned about their personal information falling into the wrong hands, and this concern is well founded. Other concerns arise from the fact that there were security holes in the early browsers that allowed scripts to access files on an individual user’s computer. Many of these Web user security issues originally surfaced when Netscape first introduced JavaScript. As you remember from our discussions in Chapter 8, “The Browser Issue,” JavaScript was originally introduced as a scripting language for interacting with Java. With both Java and scripting support added to the browser, bright programmers quickly figured out how to access client information on each Web user’s machine when they visited a particular site. Of course it didn’t take long for news of these security issues to spread. As security-related issues have become obvious, each Web browser has been updated with various security fixes to correct potential holes. Of course, correcting these “holes” has created more limitations on the capabilities of your JavaScript code. These issues, with respect to the user’s browser, deal with cookies, file access, and browser-window access.

File Restrictions In general, browsers prevent JavaScript code from executing or performing on the user’s machine any file input or output without the user’s permission. This security restriction was established to eliminate the risk of unwanted access to file directories and other personal information available on the machine. So overall, users can be fairly confident that no changes are occurring on their local computers outside of what they are viewing in their browser windows. Yet there are still issues to be aware of. Often, especially in an intranet environment, scripts may be loaded directly onto a user’s machine. These scripts may be designed to allow access to a corporate site, or other related areas. If you load JavaScript code that resides on your machine, the script has the ability to actually read, write, and create files as long as that function is allowed by the machine. Typically, the browser will still display a warning message indicating that this action is occurring and allowing the user to cancel out of the function, as illustrated in Figure 17.1, though how the browser ultimately reacts is determined by security settings.

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FIGURE 17.1 Browsers detect suspicious file access attempts and allow the user to stop the activity.

Messages will display similar to the one in Figure 17.1 when JavaScript, or any other code, attempts to perform file access commands outside of the browser (as shown in Listing 17.1), which if allowed by the user creates a text file on the C-drive of the user’s machine. Again, this type of access can only occur if the script is opened on the user’s machine and allowed to run.

INPUT

LISTING 17.1

File Access on User Machine (fileaccess.htm)

<script language=”JavaScript” type = “text/javascript”>

Keep in mind, the code in Listing 17.1 only works on Internet Explorer by using the File and FileSystemObject objects to create a new text file. There is similar functionality available in Netscape, but it requires interaction with Java and LiveConnect. Again, with Netscape the user must agree to allow the script to execute, as shown in Figure 17.2.

ANALYSIS

FIGURE 17.2 Netscape also requires user interaction before scripts requesting File I/O can execute.

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Cookies Cookies have become a concern for many Web users primarily because of their concern about the ability of someone else to store unwanted information on their machine. Most cookie concerns are unwarranted, as you will see, because the access a site may have to your machine, by way of a cookie, is very limited. As we discussed in Chapter 16, “Cookies: Storing Persistent Data,” cookies provide a means for storing user-specific information on the user’s machine that can be accessed the next time the user visits your site. The cookies are tagged with the URL of the Web site that created the cookie so only that particular Web site has access to the information in that particular cookie. Cookies can store information about the user (for example, the number of visits to a site). But personal information about the user cannot be stored in a cookie without the user’s acknowledgment. For example, cookies can be used to store form information for sites the user frequents. If a cookie is used, the next time the user visits the site, the form information can be pulled from the cookie and used to auto-fill the form. Keep in mind though, the cookie file stores the data in plain text; therefore, any personal information stored in a cookie, such as a credit card number or password, can be read by anyone who has access to the machine. Therefore, sensitive information is typically encrypted or encoded in the cookie, making it difficult to read. For example, the following illustrates a sample cookie file contents: uniqueUserId %2A0AF A567B%2D1F91%4122D8%12C3CB4DA%31D2F839A51BF%8E jstest.com/ 35448444887922553694551245084014324637686*RMID de9123357433bf7f5e30 jstest.com/ 700513675670403730714378214452525642544240029455*

As you can see, by using encryption techniques you can make it difficult for others to interpret the information you store in the cookie file. Keep in mind other Web sites cannot access the cookie files that you create—only those individuals with access to the actual machine can view the cookies on the machine. There are some general restrictions placed on cookies by browsers to protect the user. For example, in Netscape, your Web site domain cannot create more than 20 cookies. If you attempt to create more, one of your existing cookies will be discarded when you add cookie number 21. Although this restriction does not exist in Internet Explorer, to eliminate potential errors you will not want to create more than 20 cookies. To avoid this problem, I would recommend creating only a couple of cookies and storing all necessary information in them. Keep in mind that there is a size limitation on the cookie description of 4096 bytes. Finally, there cannot be more than 300 total cookies on each computer system.

Privacy and Security

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For more specific information about creating and accessing cookies, refer to Chapter 16.

Browser Window Access As we discussed in Chapter 12, “Windows and Frames,” through the use of objects, specifically the Window object, you have the ability to access the browser window and associate elements from your JavaScript code. By doing so, your JavaScript code is capable of writing new content to the browser window and changing the appearance of the window by adding and removing window chrome. As you probably already know, you can have multiple browser windows open or view multiple URLS in one browser window if you are using frames. Although the browser window allows you to open multiple URLs on one page or even in multiple windows, you can only manipulate the content from pages on the same URL of the JavaScript code that is executing. We will look at this concept more in the next section when we discuss the same origin policy.

Computer Resource Limitations Another important security step that has been taken is to limit the computer resources available for execution of the script by the browser. These limitations help ensure that a script does not overuse any of the resources on the user’s machine. Some limitations are actually specified by the user within the browser. For example, the user can specify the amount of cache that the browser can use to load an HTML page or image. Although cookies can be created by a Web site the user visits, Netscape limits the size of each cookie file to 80 kilobytes. This forces the Web designers to limit the amount of information they can store in a cookie. One restriction that Web developers need to keep in mind is the limit on the number of instructions (or statements) that can be executed for a JavaScript script, which is one million. This is to keep a script from executing and essentially locking up a user’s machine. Typically this is not an issue, as there are not too many situations where you would need to have a script execute over 1 million statements. However, if you ever need to do complex calculations, this could become an issue. Consider the following code: var totalsum = 1; for (var x = 1; x < 1100000; x++) { totalsum = totalsum + x }

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This code would not complete its calculation because 1,100,000 is greater than 1 million. In order to perform a complex calculation such as this, you would have to break it into smaller calculations. Doing so would keep the browser from rendering the system unavailable to execute the script.

Privacy for Webmasters Although a lot of steps have been taken in the browser world to protect the Web user, very little security exists for the code you write. In fact, because JavaScript is a scripted, and not a compiled language, everyone has the ability to see exactly what your code does. Yes, unfortunately the JavaScript code that you create is not secure. As you have probably learned by now, you can pretty much view all the source code used on pages on the Internet by simply selecting the View Code option in your browser for a Web page. So, you know those cool JavaScript functions you create, they can be copied by anyone. There are some who would tell you that you simply need to create a separate .js file in which to store your JavaScript code. That way, when the user views your HTML source, they simply see a reference call to the .js file that contains the corresponding JavaScript code. <script src=”myjavascript.js”>

As we have previously discussed, if you place JavaScript code in a .js file the source does not display when the user selects View Source for the HTML page. But they do see the src= statement indicating the name of the file on your Web site that contains the JavaScript code. This file is not protected or hidden, so a curious Web user simply needs to type in the name of that file with the URL and they will have access to the source. Keep in mind, the use of the separate JavaScript file was never designed for security or privacy purposes, it simply cuts down on your development work by placing frequently used code in one central location. There are methods that can make your code a little more difficult to interpret; but, quite frankly, most methods just cause more grief than they are worth. However, if you are really concerned about the privacy of your code, you may want to consider using a different method. One way to make your JavaScript code more obscure is to place the entire script on one line within your browser. I am going to attempt to illustrate how this looks, in Listing 17.2. Unfortunately there is a limit to how long a line of text can be in a book, so the code will appear on several lines even though it would be one continuous line.

Privacy and Security

INPUT

LISTING 17.2

571

Obscuring JavaScript Code (oneline.htm)

<script language=”javascript” type = “text/javascript”>

Microsoft

Netscape

W3C

Sams Publishing

If you look at the code above, the JavaScript portion of the code is crammed together, making it more difficult to read. (Although, any astute Web developer can figure out how to space it all out again.)

ANALYSIS

Another method you can consider is not only placing the code on one line, but also changing the names of variables and functions so that they do not relate to their actual functionality. For example, instead of having a function called selectlink, why not call it feedthedog? Okay, now you can really have some fun. Although keep in mind, you will have to maintain this code, so don’t make it too difficult for yourself to figure out. You should actually keep an unaltered version for yourself.

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If you are really concerned about protecting your code, you can consider using a codeshrouding program. These programs read your code and translate it to source similar to the original source by removing all of the formatting and then using randomly generated variable names. Again this code still can be deciphered, but it does take some work. Finally, you can add copyright statements to your code. Although this does not inhibit someone from copying your code, it does give you some legal grounds if you need to prove it was copied. Of course, you will have to determine if it is really worth your time and effort to pursue.

Note

The issues we have discussed so far have dealt with the user’s ability to view and copy your JavaScript code—that is,the code that runs on the client-side. Server-side scripting—JavaScript code that runs on the server—is never passed to the Web browser. Therefore, this code remains unseen by the user.

Same Origin Policy In an effort to ensure that another site cannot manipulate your own content, the same origin policy was added to browsers. What exactly does that mean? As you know, you can have documents from multiple locations open simultaneously in a browser window using frames (as we discussed in Chapter 12). These documents can come from any valid URL. You also may remember from that chapter how a script in one frame can interact with the contents of another frame. This will work fine if the documents are from the same URL, but if they are different the interaction is not allowed. The origin of a document is determined by the protocol (that is, http or ftp) and domain. If these values match for both frames, then the script in one frame is allowed to interact with the other frame’s contents. If you attempt to modify a document from a different origin, an error message such as “access disallowed” or “permission denied” will display in the frame. Okay, this may not sound like such a big deal. After all, this ensures that someone cannot alter your Web site content . But, imagine your organization has multiple URLs, or is using different protocols on the Web site (for example, in a situation with an Ecommerce site where the order form is secured and the protocol is https:. Because the rest of the site has an http: protocol, you will have a difficult time interacting between frames, even though both pages are on the same server. To allow for the fact that you may need to access different portions of your site using different protocols, you can set the document.domain property to match. In order to use

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this property, both pages must exist on the same domain, but they can be from different servers or have different protocols. To do this, you simply need to set the property to match for each document: document.domain = “www.testdomain.com”

Keep in mind, you can only set the domain value to match your current domain. In other words, the domain value cannot be modified from another domain. Of course, the document origin is not the only check performed by the browser. The document object model for windows or frames that do not have the same origin cannot be accessed. Essentially any object, property, or method that either grants access to the user’s local system or provides information about Web browsing activity, such as the Location object, cannot be accessed if the origin does not match.

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Privacy Impact on JavaScript Based on our discussion so far, you can see that although security measures have been taken for JavaScript, there are still things for the developer to consider. For starters, all documents on your Web server are accessible if the user knows the name of the file. In order to keep users from accessing sensitive files, you need to have some type of security established for the documents that you do not want all Web visitors to view. The best method for doing this is to employ some form of password protection for the documents with content you want to restrict. We will discuss password protection options in the next section. Another issue you may want to look at is creating signed scripts. As Web users become more leery about running scripts on Web sites, many of them are opting to disable the running of scripts within their browsers. In an effort to gain the user’s trust in your site and the script that is running on it, you may want to look at signing your script. Unfortunately this ability is not supported equally by Netscape and Microsoft. In fact, Netscape is the only one that offers the ability to truly sign your script. Microsoft offers a capability called Authenticode, but it cannot be used directly to sign scripts. Since Netscape’s object signing options work well for signing your JavaScript code, we are going to discuss the process of setting this up in this section below.

Password Protection Depending upon the type of Web site you are creating, more than likely you do not want all of your content accessible to everyone. You may want to restrict certain documents so that only users with a valid username and password can access them. There are multiple methods that you can use to password protect your site, and the methods you can use

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ultimately are based upon the capabilities of the location where you are hosting your site. There are three general types of password protection: using the operating system, a CGI Program, or JavaScript coding.

Using Operating System Security Each networking system has security that can be used to limit the access to particular directories. This same security can be used to limit access to a particular Web directory. For example, with Windows NT/XP as your Web server, you or your network administrators can create a sub-directory for your Web site where only certain users have access. When a user attempts to access a document from that directory, they are requested to specify network login information in order to access that directory. Once they have specified the appropriate login information they can access any files within that directory. This type of security can be specified for multiple users, and is probably one of the more effective methods of security because the login is required in order to access any documents within the corresponding directory. If the documents you want to secure are sensitive, you definitely want to consider using either this type or CGI security options outlined in the next section. You can contact your Network Administrator to determine the feasibility of incorporating this type of security. Again, if your site is being hosted, this type of security is probably not available.

Security with CGI Scripts If the server that hosts your Web site supports CGI scripts, this is probably the best method for securing your documents. But if you select this method, you also need to either create the CGI script that verifies the username and password information or find one that you can use on the Internet. Unfortunately with the complexity of CGI, it is not feasible to look at creating a CGI script within this chapter. If you decide to pursue this path, there are several resources available for learning more about CGI, including CGI scripts that you can download from other sites. You can find CGI resources listed in Appendix D. Another great source for information on CGI is Sams Teach Yourself CGI in 24 Hours by Rafe Colburn.

Password Protection with JavaScript Let’s look at some options for performing password protection within JavaScript scripts. Keep in mind, as with any type of programming you do, there are many options when determining the best method for accomplishing the task. With JavaScript, you can even create encrypted passwords. Potentially the user will be able to view your code, so you will want to use a method that is least obvious. For example, look at the code in Listing 17.3; although it provides a method for specifying a password, if users view the

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source code they immediately can see that they simply need to open the file named pswdfile.htm.

INPUT

LISTING 17.3 Simple Password Protection (password1.htm)

<script language=”javascript” type = “text/javascript”>

The code for password protecting a file in Listing 17.3 provides a quick and simple method to request a password from the user with the prompt() method as shown in Figure 17.3. If you examine the code closely, you will see that the user must specify the name of the HTML document as the password. This works well except for the fact that you must verify that a valid password was entered in order to open the file. Therefore, if you view the code you can quickly see that the password is specified in the if statement. With this code, the user only gets to see the document if the correct password pwrdfile is specified. Otherwise, an alert box displays indicating the password was not correct and the current document remains displayed in the browser window.

ANALYSIS

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Tip

Another way to make the code even more secure is to remove the if (docpwrd == “pwrdfile”) statement. This eliminates the inclusion of the

password from your code if the user decides to view the source. Again, the only way the appropriate file is opened is if the proper password or file name is specified.

OUTPUT FIGURE 17.3 Using the prompt() method, you can require that a specific password be entered before a document can be opened.

Note

We discussed the prompt() method in Chapter 12 when we talked about popup windows.

As I mentioned earlier, JavaScript coding works well for the general protection of your documents, but it does not ensure that sensitive documents could not be accessed by an ambitious visitor. Another method you can use to limit file access is to distinguish (encode) the password, as shown in Listing 17.4.

INPUT

LISTING 17.4

Encoding the Password (password2.htm)

<script language=”javascript” type = “text/javascript”>
Privacy and Security

LISTING 17.4

577

continued

{ var docpwrd = prompt(“Specify the password for this document:”, “”); for(i =0; i < docpwrd.length; i++) { enterpwrd += docpwrd.charCodeAt(i); } if (enterpwrd == checkpwrd) { self.location = docpwrd + “.htm”; } else { alert(“That is not the correct password.”); } } //-->

The code in Listing 17.4 is very similar to the previous password code in Listing 17.3. The only difference is that we used an encryption method that converts the password entered by the user to a numeric value. That numeric value is compared to the stored numeric value of the password. If they match, the code opens the file, as we did in Listing 17.3. We are creating the numeric value by simply converting each character in the specified password to its two-digit numeric value with the charCodeAt() method of the String object. Of course this method is still not totally secure, but it requires some work to determine the password. There are actually several different methods that you can use to encode values within your JavaScript code. In fact, there are several different examples of methods that can be used on the Internet. Many of the sites referenced in Appendix D have samples of encryption methods.

Creating Signed Scripts in Netscape As we mentioned earlier, Netscape provides the capability to digitally sign your scripts. Digital signatures assure a user of the reliability of the script(s) that reside on your Web

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site. When you have a digital signature attached to your script it essentially tells the user that your script can be trusted because it has a valid signature attached. By the same token, allowing a signed script access to the user permits elements of the browser that are not typically accessible by your JavaScript code; specifically, you no longer need to worry about the “same origin” policy we discussed earlier in this chapter.

Note

Keep in mind the concepts related to signing scripts discussed in this section are only relevant to Netscape. Although Microsoft has a version for digitally signing called Authenticode, it is not available for signing your JavaScript code. For more information about Authenticode, visit the Microsoft Web site.

In order to sign your script, you first create the JavaScript code that you want to sign. Once your code is complete you run a signing tool (such as the Netscape signing tool) to create the digital signature. The signing tool creates and associates a digital signature with the script you want to sign. The digital signature is placed in a Java Archive (JAR) file. Once you create your digital signature you cannot alter the script code. The signature stands to confirm the validity of the code, if the code is altered then it is no longer valid. Also, remember the user may refuse to allow your code to run, even though it has a digital signature. Or the user may be running a non-Netscape browser meaning the signed script is not valid. You need to allow for both of these situations when designing your script.

Obtaining a Digital Certificate In order to sign your JavaScript code, or any other code that you develop, you must first obtain a digital certificate. The digital certificate, or digital id, is a code that is assigned to your organization that identifies you as a trusted source. When your JavaScript code is downloaded to the user’s machine, the digital certificate is also downloaded. When the HTML document containing the digital certificate is loaded, the user is notified that JavaScript code has been loaded and it is requesting additional access; the user has the option of either allowing or rejecting the request. Each digital certificate is issued by an organization called a certificate authority. The digital certificate is issued when you provide specific information identifying you as a valid certificate holder. There are various organizations on the Internet where you can purchase a digital certificate. The most popular is VeriSign Inc.(www.verisign.com). There is a fee required to obtain a digital certificate in addition to the submission of documents

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to prove your identity. Once you have obtained the digital certificate you can assign it to the scripts that you want to sign, as outlined in the next section.

Signing a Script Once you have your digital certificate, you are ready to sign your scripts. In order to sign your script, you will need to obtain another program from Netscape called SignTool. This utility allows you to sign your JavaScript for NetScape. You can find this utility at http://developer.netscape.com/software/signedobj/jarpack.html. This page includes the information you need to not only acquire this utility but also the steps you will need to take to package and sign your scripts. This file creates a Java Archive file, .jar. A .jar file is a compressed file format required to work with Netscape Navigator security. The .jar file contains a file that lists the files within the .jar file, the certificate information, and the contents of the file when it was signed. This is how the digital certificate ensures the JavaScript code is secure. If the JavaScript code has been altered, then it does not match the contents of the .jar file. In order to specify which script should be signed, you need to add two additional attribute values to your Script tags: the Archive and id attributes. The Archive attribute must be added to the first Script tag that you want to sign. If the Script tag is not near the top of your HTML document, you need to create a Script tag to specify the attribute value. This attribute specifies the name of the .jar file you want to associate with the HTML document. For example, if the .jar file was securejs.jar you would specify the following: <script language=”javascript” type = “text/javascript” archive=”securejs.jar” id=”sec1”>

When you run the SignTool utility it assigns the attribute specified by the Archive attribute to the .jar file. When the user loads the HTML document, the attribute serves to point to the corresponding .jar file. As I mentioned earlier, the other attribute that must be specified is the id attribute. This attribute must be unique for each script associated with a particular .jar file. When the SignTool assigns the .jar file it generates a unique value (hash value) and associates it with the particular script. Once the user loads the document in his browser, the id attribute is again used to compare the hash value with the .jar file. The value of the id attribute can be any valid string, as long as it is unique for the corresponding .jar file.

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Summary This chapter discussed the various security issues you need to be aware of when developing JavaScript code for your Web site. Security is a concern for site visitors and hosts alike. Because of security “holes” that were exposed in early versions of browsers that allowed scripting, measures were put in place by Netscape and Microsoft to create a more secure environment for the browser user. These security measures were related to cookie access, file access, and browser window access. All three elements were discussed in the chapter. Ensuring privacy for the site you are hosting is a bit more difficult than simply protecting browser users. For the most part, the entire contents of an HTML page, including your JavaScript code, can be accessed and read by anyone. In order to mitigate this security risk, we discussed how you can secure documents through the operating system on the Web server by using a CGI script or by creating JavaScript code to password protect the page. Finally, in this chapter we looked at the exclusive Netscape method of signing scripts to allow you to have additional access to the user’s browser.

Workshop In the workshop today we will use the questions to review what you have learned in this chapter about privacy and security when working with JavaScript.

Q&A Q. How secure is a file if I password protect it by requiring the file name to be specified? A. Within the chapter we looked at a couple of examples that illustrated methods for password protecting a particular file by requiring the filename to be specified in order to open the file. If you use this method the only real security is the fact that the user needs to know the file name in order to open it. But keep in mind, if they just type that file name in as part of the URL, it will open anyhow. This method only serves to help hide the file from most visitors. Also, be sure that the file name is not hidden within your script. If you really want to ensure that specific files are secured from unauthorized access, your best method is to use a CGI script or use the security of the operating system running the Web site to secure the files.

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Q. How do you use the operating system to secure a file? A. If you have access to the computer that is hosting your Web site, operating system security is a good option. In order to use this method you would place the files that you want to secure in a separate folder within the Web site. Once you do this, you can set the security for the folder to only allow specific usernames and passwords for access. This method requires you to have Administrative privileges for setting security. Q. How does the Same Origin Policy affect my Web site? A. The Same Origin Policy simply means that if multiple documents are open, they must be from the same original URL in order for one document to interact with the elements of another document. This does not inhibit you from opening documents on other sites from your Web site, but you will not be able to interact with those sites.

Quiz 1. What are the cookie limitations imposed on cookies? 2. Does a separate .js file help to secure your JavaScript code from the prying visitor? 3. What is the advantage of creating a signed script?

Quiz Answers 1. Within Netscape Navigator, a Web site domain cannot create more than 20 cookies. If you attempt to create 21, an existing cookie will be deleted. There cannot be more than 300 total cookies on the system, with a maximum size of 4096 kilobytes for each cookie. 2. Only a little. Although the JavaScript code you create does not appear directly within the HTML page when a user selects to view the source, it still indicates the name of the .js file. The user can view that source by simply typing the name of the .js file as the path for the browser. This method is intended to be used to place frequently used JavaScript functions in one location so they do not need to be specified on each page. 3. Signed scripts allow you to identify yourself to the visitor as a trusted source. By doing so, you also request that the user allow you to have additional access to his system. He has the option of allowing or rejecting your request. This capability is really only available within Netscape, so you need to code not only for other browsers that don’t provide signing, but also for the situation where the user rejects your request.

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Exercises 1. Create a page that successfully executes over a million mathematical calculations without being stopped by the browser. Keep in mind, you need to break the calculations into smaller steps. 2. Create a page that requests a password before allowing the user to view a specific file. The user should only have three attempts to enter the password. Once the maximum number of attempts has been tried, an Alert box displays a message. 3. Create a page you can use to create your encrypted passwords. Basically you just want a page where you can type the desired password and it returns the encrypted version.

WEEK 3

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18

Plugins and Applets With all of the capabilities of JavaScript, there are many additional features that can be used to enhance the capabilities of your Web page. You can enhance your Web site by using different Java applets and plugins. Java applets are essentially Java programs that have been designed to run from a Web page. You can use JavaScript to interface with the applet (for example, starting and stopping) and change its properties and methods. Plugins are similar to applets in that they allow you to add additional capabilities to your Web site. A plugin, however, is not necessarily developed within Java. There are many commonly used plugins that are available for use within your Web site. Typically these plugins are available for viewing multimedia type files. For example, Adobe provides a plugin that allows Web users to view Adobe Acrobat files within their browsers. This chapter looks at plugins and how you can detect their existence. Of course with some file types, you may not be as concerned about the existence of a particular plugin as just knowing whether or not the file type is supported by the browser. We will also look at how to determine if the browser supports a particular file type. Finally, we will show how to work with Java applets and how they can be incorporated into your Web page.

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This chapter will teach you about: • Plugins versus applets • Detecting plugin installation • Working with plugin objects • Working with applets

Plugins Versus Applets Plugins and applets are terms that are frequently used interchangably, giving a novice user the impression that they both refer to the same thing. However, in fact, they are two different things—although, browsers treat each similarly. An applet is essentially a mini-application created within Java that will run on any JavaEnabled browser. Once called, a Java applet is capable of interacting with the user without any interaction from your HTML or JavaScript code. Although, ideally, you want to use JavaScript to create an interface between your Web page and the embedded Java applet. By doing so, as the Web developer you have control over the interaction with the Java applet. Plugins on the other hand have a much broader scope. A plugin is essentially an add-on to the browser that allows the user to work with additional types of data within the browser. Typically, browsers enable the playing of sound files, video clips, or the ability to view specific file types. For example, Adobe Acrobat files are frequently available on Web sites to provide copies of standard printed documents, as illustrated in Figure 18.1. By loading the Adobe Acrobat plugin, a user can view these files in the same format and layout in which they were designed. FIGURE 18.1 Plugins allow you to incorporate other file types into your Web site.

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Plugins and applets were both originally introduced by Netscape with the use of JavaScript to communicate with them. This technology was originally called LiveConnect and was first implemented in Netscape Navigator 3.0. Microsoft followed suit by adding the capability to Internet Explorer also in 4.0, although they were not able to use the LiveConnect name that was trademarked by Netscape. The Applet and Plugin objects are now part of the document object model (DOM) maintained by the W3C, which we covered in Chapter 8, “The Browser Issue.” The Applet object is a child object of the Document object. In HTML, the code for the applet is enclosed in the tags. But you can reference an applet directly from your JavaScript code by using the corresponding properties and methods for the object, as we will discuss later in this chapter. Netscape Navigator provides a child object to the Navigator object called plugins. This object is actually an array of all the plugins that are installed on the browser. Using the properties associated with the plugins object you can determine specific information about a particular plugin, such as its name or description. When working with Internet Explorer, the plugins object associated with the Navigator object is not supported. Internet Explorer treats plugins like ActiveX controls and therefore you will need to use the Embeds object. We will look at this in more detail in the chapter, but it is important to realize that each browser handles plugins differently.

Detecting Plugin Installation As we have already discussed, plugins allow you to add multimedia capabilities to your Web page by adding the ability to play sound, video, or just display specific file types. However, in order to use these types of files, the plugin must be installed on the user’s browser. Therefore, you need to determine whether the plugin, or at least support for the desired file type, exists before attempting to load it on the page. There are a multitude of plugins available for incorporating this type of capability into your Web page. Some plugins allow you to interact with them, as we will discuss in the next section, while other plugins simply perform a specific task with the only interaction being the specification of which file to open. Typically as the Web developer it becomes your responsibility to ensure that the files are saved in the appropriate format for use with the selected plugins. For example, in order to open a PDF file you must have the appropriate file saved in that format. Although the plugins are generally free to download by the Web user, the software to create the file formats typically needs to be purchased. Table 18.1 lists some of the most common plugins used and the Web sites where you can gather more information about using the particular plugin.

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TABLE 18.1

Commonly Used Plugins

Plugin

Description

Adobe Acrobat Reader

www.adobe.com

Windows Media Player

windowsmedia.com/download/download.asp Allows you to view multimedia files. Available as default player on most versions of Windows.

Apple QuickTime

www.apple.com

Netscape LiveAudio

home.netscape.com Provides the ability to play AIFF, AU, MIDI, and WAV files within Netscape Navigator.

RealPlayer

www.real.com

Allows the Web site visitor to view files stored in a .PDF file format. Storing a file in a PDF format allows you to preserve all formatting.

Typically used to play animation and video files.

Provides the ability to play streaming audio and

video. Flash

www.macromedia.com/flash

Macromedia’s premier plugin that allows users to view animations, presentations, and any other interactive Web content created using Macromedia Flash.

Shockwave

www.macromedia.com/shockwave

Provides the ability to view animations, play games, and run interactive demos.

As you can see by the list of plugins in Table 18.1, there are several multimedia players available to play audio and video files. For example, current versions of Microsoft Windows come with Windows Media Player installed, but the user can choose to load another player, such as RealPlayer, as their selected multimedia player. Because of this, you may not always check for the availability of a specific plugin but the support of a specific file type by the browser. We are going to look at both techniques to determine if a plugin is loaded and to see if a file type is supported. Depending on what you are trying to accomplish, you can determine which type of testing works best for your Web page. Of course, the type of checking you are going to be able to perform is constrained by the user’s browser. While both Netscape and Internet Explorer support the use of plugins, the way each browser handles plugins is different.

Plugin Handling by Browsers As we briefly mentioned earlier, each browser provides the Navigator object which contains specific information that describes the specific browser being used by the user. If you remember, we used the properties related to this object in Chapter 8 to determine which browser the user was using to view your Web site. This object also contains two child objects, shown in Figure 18.2, plugins and mimeType, that provide information

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about the specific plugins that are installed within the browser. As shown in the figure, these objects are actually part of plugins and mimeTypes arrays that contain corresponding objects for the browser. Unfortunately, only Netscape makes use of these plugin-related child objects. FIGURE 18.2 Navigator

The Navigator object provides child objects for handling plugins. Plugins

Plugin

Note

mimeTypes

mimeType

Multipurpose Internet Mail Extensions (MIME) is the standard used to classify different types of files and to transmit them over the Internet. Each type of file is called a MIME type. By default, most browsers have a very limited number of mime types that they support (for example, HTML, GIF, JS). As you add additional plugins to your browser, the number of MIME types supported within the browser increases. Because some multimedia plugins support multiple MIME types it is highly likely that users may be able to view a particular file type even though they are lacking a particular plugin.

As you will see in the next section, within Netscape it is quite simple to check the plugarray to determine if a particular plugin has been installed on the user’s machine. If a corresponding Plugin object does not exist, you can then inform the user of the need to load the required plugin.

ins

Although within Internet Explorer, the Navigator object also includes the Plugin and objects, they are not supported in the same fashion as in Netscape. Microsoft uses a different methodology for dealing with plugins when working in the Microsoft Windows environment. Internet Explorer treats all plugins as ActiveX controls. Although they are actually an ActiveX control, plugin type controls are still commonly referred to as plugins, even in Internet Explorer. mimeType

Because plugins are handled as ActiveX controls, they are added to a page using the